Component parts and Assembly
ECU/MAFS Problems
Breakdowns, Symptoms, Cure, Cost

For a discount on the cost of your MAFS repair see below for details.

Lofty ,
A friend of mine has the W169 and had the P0065 (I think) which indicates the MAF sensors (this one has two, one on the cat converter & one on the inlet manifold). His engine light has been on for 2 years now, he finally took it to a local garage who replaced both for a cost of nearly $500 but still the engine light stayed on.
I then did a quick google search and suggested that we clean the throttle body. I reset the fault code and the light has never come on since.

More recently (yesterday to be precise) I decided to clean the throttle body on one of my own A class, which for over a year now has been stalling intermittently, mostly when you slow down at a roundabout or stop. I have never had the engine light come on but was always a bit funny when it returned to idle from normal driving, it would dip below 500 revs and then back up to 700 and then often stalled, which is actually dangerous as you lose brakes & power steering.

Again a quick google and a look at your site and all the horror stories about the need to replace the ECU at great cost (incidentally, the garage of my friends also suggested that be the next port of call if the ECU as the MAF sensor replacements failed to resolve the issue) So, I cleaned mine as well (Also had lots of goo around the butterfly rim) on re-staring, after approximately 5 minutes it settled down and is idling just perfectly.
the engine revs are no longer dipping below 500 revs, instead settling down to 700 odd immediately!
I have done at least 50 km since then and not a hint of stalling!

So, I would strongly suggest that the throttle body be cleaned as a first port of call,(First Job) it costs very little, only takes around 30 minutes to do and as I have now experienced twice solved the issue. It makes me a bit mad as these garages and Mercedes Benz should know that this can be the real cause but just want to make money from owners who do not know any better!
Regards, Eugène 3.8.13

Eugene ,thanks you for the information.
ECU cleaner which can also be used for cleaning the throttle body can be purchased off e-bay UK for as little as £10.00 a can and is a good investment, the sensors referred to in the E-mail on the W169 are the upstream and downstream lambda sensors, which monitor the fuel and exhaust gases as they progress through the inlet and exhaust systems.
It is they that alert the ECU to the fact that the combustion mixture is incorrect,or unbalanced, for what ever reason .
In the cases mentioned above because the throttle body was not able to work as intended due to contamination and build up of fuel deposits the problem arose.

This page gives details of a breakdown I experienced with my 'A' Class, along with the Cause, History, Cure, & Cost.

* October 24th 2005. 'Mobilo-life' called due to stalling and rough running ECM (Engine Control Module) (Hand book ref.= Engine diagnosis Indicator lamp) was lit. 'Mobilo-life's' diagnostic's detected a defective oxygen sensor, located in the catalytic converter,
*Engine running normally, stalls on approach to roundabout,
* rolling re-start no problem, engine sounds rough,
* No dash warning lights at this point.

* Stopped for twenty minutes engine re-starts no problem,
* 500 yards engine sounds rough, acceleration erratic
* Engine Control Module lamp illuminates on dash display.
*Engine sounds as if is misfiring,
* black thick fumes omitting from exhaust thereby giving an indication that the fuel to mixture ratio is far to rich
* Engine continued to run and I made it home 1.5 miles away.

Initial diagnosed Cause There are two oxygen sensors both of similar appearance the one nearest the engine 1 is the one which the 'Mobilo-life' technician diagnostic equipment says has failed.
So what is the function of the oxygen sensors
"Quote From NTK oxygen sensors.
NTK is the brand name for oxygen sensors manufactured by NGK Spark Plugs.
NGK is the world's largest manufacturer of oxygen sensors with over 10 million sensors produced annually.
Oxygen sensors are used in modern automobiles to control the fuel and ignition systems to optimise a car's performance in the areas of emissions and fuel economy. Sensors are located before and after the catalytic converter to check on the amount of oxygen in the exhaust. The sensor sends signals to the car's on-board computer, which then can adjust several variables, including air/fuel ratio and timing, in order to bring the engine into the optimum operating range Unquote

The car was booked into an Independent Mercedes-Benz specialist www.mercedesj-a-haynes and I got an appointment for the repair to be carried out 4 days later during which time I did not use the car, However when the appointed garage tried to order the part, only the sensor at the back of catalytic converter was available. 2 The other is only available on a special order from Mercedes-Benz and the reason why it is required has to be specified along with chassis number etc, as it is not normal for it need replacement!

Action the garage therefore purchased the sensor available, (the sensor at the back of the cat. 2 Part Number A 002 540 01 17 (cost £149.69 inc (26/10/05

Availability of 02 sensors at greatly reduced prices.
For supply of replacement 02 sensors Lambda Sensors see page 34 09/06/09 this information was kindly provided by Tony, a fellow owner.
I digress for good reason at that point because Mercedes Are now charging well over £150 per sensor.

However When I got the car out of my garage on the appointed day it started as normal and ran fine, although the engine management light was still displayed.
I arrived and left the car for the garage to fit the oxygen sensor and clear the fault, however when I returned I was told they had discovered further but related reasons as to why the ECM light was lit. a second diagnostic test had revealed that the mass air flow sensor had gone defective.

However if you have what appears to be a MAF Sensor problem Check that the air filter has been installed correctly and that the filter has been fully housed with filter lock fully slid into position this can cause the same symptoms as a MAFS defect as the air coming in the filter housing by passes the filter and flows direct to the MAF sensor.

These components ECM ( engine control module A and MAFS ( mass air flow sensor) B are integrated on the 'A' Class and therefore a fault developing in one requires them both to be replaced, at considerable cost, £764.05 plus VAT making a total bill of £897.76, a lot of any bodies money and I'm not amused.
I paid the garage for their time, diagnostic test and the part they had purchased on my behalf, they not being able to return it to MB Brighton, and in view of the fact that the car was still running normally drove it to my usual MB Garage, Main Dealer, who seeing the diagnostic sheet from the Independent Mercedes-Specialist confirmed that the fault was the Mass Air Flow Sensor.( MAFS) I had little choice but to order a ECM/MAFS combined unit which was going to take about 5-7 days to arrive, it being coded to my car at the factory in Germany

Having received this confirmed diagnoses I got the MB Garage to contact MB and ask them for a goodwill payment towards this cost, I received a direct refusal on parts and labour.
Having now driven some 50 + miles without a problem, apart from the ECM light showing on the dash display I still wasn't confident that the information I was being given was correct, and as I'd had to pay for the oxygen sensor I decided to fit it.

I then attended my local garage, the owner of whom I've known for years, he did an omissions test for me and the car was fine and would easily have passed an MOT. I then went back to the Specialist Mercedes Garage and asked them to delete the fault and then road test, sure enough the ECM light re-appeared after about few minutes of driving. That confirmed conclusively that it wasn't the sensor but that the fault was positively in the ECM/MAF. sensor.

ECU/MAFS or lambda?
Over the past few years it has become very apparent that diagnosing the problems of fuel mixture controlled by the ECU/MAFS and a faulty Lambda sensor in the catalytic converter along with in some cases a sticky/dirty throttle body also being implicated is difficult
In my case the lambda senor was diagnosed by Mercedes for the engine fault that existed, when in fact it turned out to be a defective MAFS, in other case it turns out to be the other way round.

Where an ECU/MAFS has been responsible and causing the car to run rich or weak for some time it is understandable if the lambda then shows up as being defective and as result may have to be replaced after the ECU/MAFS has been repaired.

However Lambda's can go defective as can all components with out good cause which make detecting them even harder

Doug one of my readers had a running problem, so he had the car put on the Mercedes-Benz 'Star' diagnostics only to be told the his ECU needed replacing cost £1300
, I'll leave Doug to tell you
Yes Lofty, after the original diagnosis and £1300 quote from MB, I found your site and link to ECU Testing, I sent it of to them and they contacted me,
Latest Mercedes-Benz quote to a owner was £1400.00 30/09/10
They said it wasn't faulty but I asked them to rebuild it anyway.
The car drove fine for nearly two months before the symptoms showed up again with the car cutting out and running very erratically at low speeds, and the engine management light coming on and staying on.
ECU' Testing's work being guaranteed for two years I returned it to them, the unit went backwards and forwards twice in the space of a few days.
The first time back, the car was worse than ever. The second time they temporarily re-coded it to an A-Class they have up there, to confirm that the ECU/MAFS was not the problem (they didn't charge me any postage though).
Of course, had I gone to the independent diagnostics people first, I would have saved myself £300, but at least I have a 2 year guarantee on the ECU with ECU Testing.

Following the return of the ECU for the third time, it having been given a clean bill of health by ECU Testing, Doug looked for a company that could diagnose the true cause of his problems.
Through out this period we were in contact by mail, my diagnoses being the Lambda sensor in the cat, which is what it turned out to be, albeit the diagnostics company did also clean the throttle body which would have allowed the car to make a fresh start and although Doug had spent monies with both companies his total expenditure was still way off the figure of £1300.00 he was quoted by Mercedes-Benz with no chance of him getting his money back as the new ECU would have had to be coded in Germany to his car, having been coded it would not then work on any other vehicle.
The ironic thing is that if Doug had gone along with the Mercedes-Benz diagnoses and allowed them to fit a new ECU with integral MAFS the problem would still have been present and would have resulted in him to spend further cash to rectify the true cause by fitting the new lambda anyway.
I'm pleased to say the car is now fine, Doug is happy with the outcome, and thank you to ECU Testing for going more than the extra mile, in not only re-testing once but twice to assist this customer overcome his car's problems.
Mercedes Workshops are unlikely to undertake this service on ECU's which have been modified by the fitting of additional components in the repair of the ECU

ECU do not alter the specification or fit additional components when undertaking the repair on your defective ECU/MAFS. This is not the case with some other companies advertising the repair of you're 'A' class ECU.
And even better news 01/11/2010 the cost is now only £145.00 + Vat

This photo shows the crankcase breather pipe which is attached to the bottom n/s corner of the ECU/MAFS 1 interconnected with the pipe work of the ECU/MAFS, 2 this can be found full of oil/water brown emulsion, as shown below, and is best cleaned out before an oil change if, and when the need arises.
If this breather tube is not connected to the ECU,is split, perished, damaged in any way it will cause starting, and running problems and will cause the engine management light to stay on when the engine is running, The fault picked up on a diagnostics tool will almost certainly indicate a defective MAFS because the mixture reaching the engine in not correct . So check this before even getting your diagnostics test done.

This small sample 3 is typical of the oil/water brown emulsion that can be found in the neck of the oil filler and also the crankcase breather pipe, more often than not caused by too many short runs where the engine is not able to reach its full operating temperature.
Where this is a particular problem it is advisable to use engine flush prior to replacing the engine oil. The example in this photograph is nothing to worry about, although it was causing the owner some concern. I have seen much worse, a good long run and an oil change will normally deal with the problem.
In any circumstances if the oil is changed at the prescribed intervals, not more than twelve months there will be no problem,
Do not confuse this problem with a leaking head gasket unless you are having to regularly top up the coolant header tank with fluid or on dipping the oil the level has risen and molecules of water can be detected on the dip stick.
When ever removing the ECU check the breather pipe is clear a blocked tube will affect slow running and prevent the crankcase from venting

It was interesting to see the difference in the old and the new sensors , why the change in design I do not know I just hope it is for the good of the car and that Mercedes-Benz aren't using the 'A' Class as a test bed!

I now contacted Mercedes Customer Services myself and explained the full scenario along with faxing them paperwork to substantiate that the wrong diagnoses/fault had been made and what it had cost me, as a result of my phone call Mercedes-Benz are going to reimburse me the £150.00 cost of the oxygen sensor.

Today 3 11 05 I have had the New ECM/MAFS fitted and asked for a diagnostic report showing that the New ECM/MAFS is fully functional and that the are no faults on the car at all.
I cant say that the car runs any different, a little smoother perhaps. But of course the ECM fault lamp has been extinguished, in truth I think the car is quieter running obviously due to the fuel/air mixture.

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------It is said that as you get older you get wiser! Well in this case it is true, since spending a great deal of money on my own car I have researched and am able to recommend a company 'ECU' who will reliably repair the MAFS portion of the ECU/MAFS integrated unit and you can be assured that it will meet the same standards in all respects of a New Mercedes-Benz replacement unit.
No alterations are made to the system or casing, simply tested, defective components replaced and the complete unit re-calibrated, problem codes on your ECU will also be deleted giving your ECU/ MAFS a clean bill of health and a two year warrantee on their repair's which is at least as long as Mercedes Benz new components.

ECU are also able to undertake repairs to the ECU portion, this is great news as previously this has required a £1300-£1400 replacement unit from Mercedes-Benz, Germany,
Now I'm not only sharing this information with you but have arranged a discount as well so I hope the failure of your MAFS or now the ECU is not so traumatic and costly as mine turned out to be.

This Company is Recommended for the repair of you ECU or MAFS.
ECU Tel.0044(0)1773535638 or 07971468585
Business hours 9am to 6pm Monday to Friday, Location Derbyshire. Cost £145.00 less 15% if you mention my site, + Vat @ 20% + Royal Mail Special Delivery.

So if your in doubt as to whether your problem/fault is caused by either the MAFS or the ECU portion of your unit I would advise you to send it to them, they will be able to repair which ever is found to be faulty, if on test they find a repair is not required/necessary they will only charge £35.00 for handling, if you prefer to telephone, have the details of your ECU/MAFS to hand
Repairs undertaken by this Company are guaranteed for two years.
The Repair procedure undertaken by ECU does not require the casing of the ECU to be drilled or modified in any way.
Test equipment used incorporates diagnostics equipment that can communicate with the ECU/MAFS on the test bench and the air/fuel mixture will be correct when your unit is returned to you. No further adjustments of any kind are required and the fault code for the Defective MAFS will have been deleted.

This Company will allow you a 15% discount on the repair costs
if you mention my site 'Lofty's Homepage' this discount is generously allowed by the company and is not an automatic entitlement.

Hi there Lofty,

Information from Colin in Australia :- MAFS can now be repaired in AUSTRALIA
, Just a short note to let you know that the MAFS can also be repaired in Australia now, however at a cost substantially more then you pay in the UK. 1100 Aus $. The company is called 'Injectronics' and they are based in Victoria. However they will only deal through a third party usually REPCO which is well known to your Aussie readers.

Just dropping you a line to say well done on the website, whenever we get anyone asking us how to remove there Mercedes A Class ECU/MAF we always direct them to your website.
Every customer that reads it has commented how good your descriptions are and we are giving a lot of customers there 15% discount for mentioning your site.
There should be more sites like yours and you well deserve to do very well with it.
Kind regards Roger & everyone at

So there you have it, not only do they keep to their word and do a good job, they also appreciate your business and show it.

Hi Lofty, Just want to say thanks for all the info on your site. I had trouble with my MAFS so sent it off to ECU Testing. Phoned them on Mon and sent MAFS on Mon afternoon, it was returned by Wed morning and only cost £265 which is better than £1400 that two garages wanted. Also got 15% discount for mentioning your site. So thank you again. Gary.

9.7.09 Lofty,
Just wanted to let you know your website saved me £800 +, 1st day from fitting the repaired unit at ECU Testing, it seems to be working great and the warning light has gone off. Thank you so much.
Best regards
Ron Unquote
lets hope more readers will take this route when you encounter problems with your MAFS, a 15% discount can't be bad, I wished I known about my site when mine went duff !! sadly it wasn't written


Please do not confuse the company above with this seller ashley1987-2008( 22) who is now advertising on E-Bay. I have no knowledge of the seller, and no discounts have been arranged, also note the repair is only guaranteed for a period of one year.

BBA Reman.
09/07/2007, I regret that due to circumstances brought to my attention by one owner and my own findings following close inspection of my own spare ECU/MAFS which was repaired by BBA Reman 2 years ago (25.07 2007) and in the interest of my readers I'm no longer able to recommend BBA for the repair/re-building of your MAFS, integral of the ECU Monolith.
This decision has not been taken lightly and I have had discussions with the BBA Reman Management. my reasons are outlined lower down this page.

For owners living abroad, One of my readers has used this company for his rebuild, you won't get information on how to remove the ECU but you have that info on this site, he paid 300€ and is very happy with the repair. Discounting is not arranged with this company.

Owners please be aware, if your car goes defective with a MAFS problem and is taken to a Mercedes garage for repair, MB will fit a new ECU/MAFS unit without question they will not want you having it repaired even though the warrantee period allowed by ECU Testing is greater than that on their new unit, so be on your guard, you are the customer, it is your car tell, them what your want done and stick to your guns or pay Six (6) times more for the new unit, fitted.
The latest cost of New Mercedes-Benz ECU/MAFS sensor, fitted was £1400 that's £400 more than I paid, so the repair option has to be considered by those that are aware!!

There is no doubt that this monolith of MAFS/ECU combined is a complex unit, if you intend having it repaired then it is best left alone , this does not prevent you inspecting the reeds as this can be done by carefully removing the plastic sleeve from the induction tube of the unit, see below

Before attempting to remove the ECU/MAFS combined unit it is wise to disconnect the battery.
Disconnect the negative terminal followed by the positive.
When replacing fit the positive terminal first followed by the negative
Show me more. (place your curser over the diagram to enlarge.
Use BACK to return to this page.

How do I remove the ECU monolith.

Removing and replacing the secure ECU plugs/connectors
Ensure the battery is disconnected. The plug/connectors are secured by a sliding locking sleeve :-to unlock, lift the slid, using even pressure both side, move it in an upwards direction until it will go not further .
As the lock is moved up the male section of the plug will be partially withdrawn.
When the lock is fully lifted ease the plug from the rear of the ECU, this should be a firm fit. In the case of the lower plug/connector sleeve the lock operates in the opposite direction i,e Down

To replace ensure the sliding lock section is fully raised /offer the male plug to the female, ensure the lugs situated on the sockets on the rear of the ECU are aligned and gently push the slider down, at the same time applying gentle pressure to the plug this action which will draw the male into the rear of the ECU.

With the plug held in towards the ECU, push the sliding lock down until fully home, as you do this you will see the plug section sliding into the female attached to the rear of the ECU
In the case of the lower plug the locking sleeve moves down to unlock. Up to lock

In both cases these slides can be very stiff, where this is the case use silicone spray to lubricate the slide and then tap gently up or down depending on the slide you are working DO NOT BE TEMPTED TO PULL THE PLUGS FROM THE ECU UNTIL THE SLIDE IS ON THE STOPS IN EACH CASE .

When re-installing the plugs firstly make sure the slide moves freely on the plug, then line the plug up with the ECU and push the slide up or down depending on which plug you are working, the slider must be on the stops to ensure the plug is fully housed. If you attempt any other method you will bend the multi pins in the socket of the ECU which may then become un-usable.

Having had the new unit fitted I now have an old but repairable unit that I can photograph and strip to show the component parts.
For comparison of the size of the components on this PCB the hole marked 1 is 5mm
The larger photo right hand side, is the actual size give or take a mm.
What a pity the whole PCB, along with it is hundreds of components, precision plastic/metal bonded moulding's and other components become redundant just because of one small glass reed! fails.

The ECM portion of the unit above does look like rocket science
The MAFS is a component connected to the Engine control Module, printed circuit board.(ECM/PCB)and is then simply sited in a air intake manifold that connects directly to the air filter housing.

Having now removed the sensor from its chamber and removed the deflector/air foil (cover) the cause of the failure on my unit can be easily seen.
The reed/film marked 1 is intact.
The reed/film marked 2 although still in position is burnt, it is very obvious that this was the cause of the failure of this unit, and also why the car still ran so well.
Neither of the reeds/films has completely broken or disappeared into the engine as I understand is more often the case; in such circumstances it is unlikely that the car would even start.

A Shows the fine glass reads located between two metal posts , in each case these should be clean, unbroken or burnt. B Shows the plastic cowling that forms an 'air tunnel/chamber ' around the reeds.

Despite the fine grill like structure over the sensor, without a purpose made air filter correctly located directly in front of the MAFS chamber MAFS would quickly become damaged.

Having now seen the fragile nature of the MAFS there is no way I would attempt to change/convert the 'air filter system' on my car, even the smallest particles of contamination or oil, finding their way onto the sensor reeds situated within the manifold/chamber could in my opinion damage the fine glass reeds that make up the Mass air sensing unit.

The grill seen in the above photo can be removed, this will enable you to see the MAFS reeds much clearer but take care not to touch the reeds/film
I used two fine flat bladed screw drivers and gently teased the sleeve out, take note of its position or mark before removal
To replace simply press back in as found, grill nearest to the MAFS.

In future and before fitting a new air filter I will put an air line over the entire filter to ensure there are no loose particles, even the smallest piece of debris or paper dust from the filter would I'm sure be enough to damage the fine glass reed as it heats up.

Cleaning the Reeds
If all you intend doing is cleaning your MAFS then you need go no further, MAFS cleaner is now available as far away as Australia (thank you Bob )and available on
E-Bay @ £10.99
including postage.
This can be sprayed into the induction tube of the ECU which will coat & clean the reeds, you may have to repeat this process several times but it should do the job if the reads are not burnt out/damaged as happened in my case. Follow the instructions on the can. Under no circumstances touch the reeds or you will break them off the metal supports, once broken the car will not run.

If you have problems and believe your MAFS is the cause of an engine running problem or your engine management light is displayed all the time, and are tempted to follow the cleaning procedure outlined on the SVC global site then stick to the procedure to the letter
DO NOT use compressed air in the area of these reeds and DO NOT TOUCH the reeds, they are extremely fragile and cannot be repaired by you the owner/mechanic .

. Link to yet more information,
SVC,MAFS maintenance/cleaning

I strongly advise owners not to strip the ECU/MAFS down unless it is scrap as was the one used on this page.

At the time mine went defective there were no companies I was aware of repairing these units and so I was forced to pay close to £1000 for a new ECU/MAFS coded to my car in Germany by MB. I have since had it repaired and have kept it as a spare part.

The Vehicle Wiring Harness is connected to the ECM/MAFS via the upper socket situated at the rear of the of the ECM/MAFS. 9 The lower socket allows connection of the Engine Wiring Harness 10 Although the official information says that these plugs are secured I could find no form of fixing on my car.

The ECM/MAFS is secured to the Air Filter housing at these three points. A

Note the sealing ring 7 which comes pre fitted on a new unit .
Note also that items 11 & 12 come pre fitted on a new unit and appears to be pre-fixed.

Seeing the components on the car makes life easier, to remove item 1 ease the top edge down and out it is held by a small lip on the top and bottom edge of the scoop.
This will then allow you to move item 2 back from the cross member , remove it from the air box housing by turning and pulling. When the resonator 5 is unscrewed the ECM/MAFS will be exposed.

Wayne a fellow owner has pointed out that the air intake hose in the photo above has in fact been put back incorrectly . Although it will fit and has no bearing on the air intake it does fit better when installed correctly.
If your replacement ECU/MAFS has/is being fitted by professionals you might like to check your installation as well.

When fully assembled the mass Air sensor manifold is hidden by the resonator/cover, secured by three screws. The air induction hose is fitted to the air vent on the front cross member. Air is induced into the air access through plastic pipe work connected to the air filter housing then through the air filter and on through the grill in the induction manifold throat and on to the air mass flow sensor. Never be tempted to run your car without a purpose made air filter correctly installed.(See page. 16. for more details)

A quick look round one of the new 'A' class models revealed what appears to be a ECM on it is own situated on the right hand side of the bulk head but no sign of a mass air sensor in the form fitted to the face-lift 'A160' 2/2001.

Although looking through my own site I see there is what looks very much like a ECM sitting on the top of the engine in the new A150. So have Mercedes-Benz realised their mistake or not? Be assured if there is a next time with my car, the unit will be repaired.

Mass Air Flow /MAF
Here's how they work. A constant voltage is applied to the heated reed or film. This film is positioned in the air stream or in an air flow sampling channel and is heated by the electrical current that the voltage produces. As air flows across it, it cools down. The heated reed/film is a positive temperature coefficient (pitch) resistor. This means that it is resistance drops when it is temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This current is changed to a frequency or a voltage which is sent to the computer and interpreted as air flow. Adjustments for air temperature and humidity are taken into consideration since they also affect the temperature of the heated wire or film.

Re-installing the ECU/MAFS following repair.
Having had you MAFS repaired and re-set obviously it needs to be replaced. This is a very easy job and no skill is required with minimum tools.
Do not let MB confuse you by saying the ECU needs to be set up to the car , this is not the case as it is the original ECU and still retains the settings. The car will not have be run during the period the MAFS was off and so no problems should be encountered when the unit is replaced However having fitted and reconnected the battery. (Turn on the ignition on All lights/icons will be displayed on the instrument cluster) and let the car stand for a minute or so, this will allow the ECU to re-establish contact with all the various components on the car.
There will be a sticker attached top your ECU /MAFS outlining this procedure. This action on your part also makes it less likely that the Engine management light will remain lit when the engine is started.
Should that be the case you will need to contact a Garage with a diagnostics tool to delete the light, NO other work is required by the garage.
If you have your own diagnostics tool, then simply delete the fault code and the light should extinguish.
However if you used 'ECU Testing' for your repair they will have deleted the fault code from your ECU as part of the repair service.
I cannot speak for other companies
Allow the car a couple of days of running to really settle down after re-installing the repaired ECU/MAFS unit.

Re-coding of the unit unlike a New MB unit is not necessary as the ECU/MAFS is being re-fitted to the same car. But please read the information above and also please remember when re-fitting the car's battery to turn the lighting switch to at least side lights, this reduced the spike of power reaching the ECU. Remembering to turn them back OFF after having reconnected, also remember the re-set your windows and the ABS/ESP.
Reset ESP/ABS & Reset Electric Windows

Note. If your car is pre 2000 it may well be that only a garage with 'Star' Diagnostics equipment can clear the fault from the ECU memory. Even if the light does extinguish the fault code for the defective ECM will stay logged until it is cleared. It will have no effect on the running of the car.
Just one final point the time taken to turn your repaired unit round by 'ECU Testing' is 3 to 4 days, it takes seven days minimum for Mercedes-Benz parts to obtain your New combined ECM as it has to be coded to your car before delivery and comes from Germany, so there is no time advantage in buying a new unit.

The connector for the Diagnostics test equipment is located next to the bonnet release in the cabin, under the front edge of the shelf just to the right of and beneath the steering wheel.

My Final bill
, to put my car right was £950.00
that does not include the cost of the new oxygen sensor fitted which is being paid for by Mercedes-Benz
Some are W.I.S and some are otherwise I will certainly have any future MAFS defects repaired.
Today 18.04.09 the same job done by Mercedes-Benz would cost nearer £1400.00

Read diagnostic reports:-

'Mobilo-life' report verbal only. (Oxygen sensor nearest to the engine.)
1st printed Diagnostic readout following Breakdown 24.10.05

2nd diagnostic test 2 11 05 following installation of oxygen sensor

Final diagnostic test after fitting new ECM/MAS.

As time goes on more information is coming available on the combined unit, one owner has recently suffered stalling of his engine erratic revs and general poor performance he also said he could hear the butterfly in the throttle control body flapping in an erratic matter . After many tests he got his car to an auto technician who diagnosed a breakdown of one of the electrical components within the ECU portion of the combined module, the fault code located was P0605

the out come was that he had to purchase and fit a new ECU from Mercedes-Benz, this had to be coded to his car by MB in Germany before delivery and cost him just on a £1000. fitted. If you suffer problems on your ECU/MAFS Contact:- ECU

DONT be tempted to go to a breakers yard, or E-bay where a number have be up for auction lately, like the ignition keys you will not be able to get the second hand unit re-coded and will therefore have wasted your money. Oh yes scrap yards and E bay sellers will sell them to you claiming they will work. They will not.

This is a question I put to one such seller on e-bay 30th May 2008 and the answer he gave in response-
MY question to the seller :-Q: The ECU /MAFS does as you say cost mega bucks, from MB that's because they are coded to the car, unless your unit is something very special yours will not work on anything other than the 'A' class is came from.
Your in the salvage business you should be aware of that before you sell something to some unsuspecting buyer.

His reply:- We sell a lot of stuff like this to companies which Reman, refurbish them, this item is sold as is for spares or for someone to get it coded to their vehicle as a replacement, but thanks for the info anyway.

At least this seller was prepared to answer the question , a number of them don't.
However the answer given is not appropriate to the average buyer, parts may be salvageable by an expert.
ECU testing will repair the ECU portion of this unit.
as far as the buyer getting the unit re-coded this is not possible through normal means and I'm not aware of any organisation or company that will undertake this task.
However it is possible for 'ECU Testing' to transfer the chip from your defective unit to another ECU at cost. That chip will have your original coding and so will work with your car, other checks also have to be made to the replacement unit to ensure it will work when you place it on your car. Just switching your chip to another ECU will almost certainly not solve your problems.

Another example of the defects that can be caused by a defective ECU/MAFS.
I think this next e-mail makes the point at just how obscure these defects can be:-

'Quote' ' This has just started in the last couple of weeks.
On a cold engine, the revs are steady for a few seconds, then die right down, then surge back up, and then repeating. After a few minutes on the drive all is OK
. If I drive whilst it is still surging I get the kangaroo juice effect of the engine dying and surging. Within a mile, or a couple of mins it is all settled down.
Car has done 89,000 miles and is due a one spanner service in 700 miles.
Any tips please, so I can point the garage in the right direction?

Following the A service on this car by MB, the plugs, ignition module, and oxygen sensors was replaced the car still ran the same, causing the owner some concern. Even a test of the 'Star' diagnostics tool failed to pick up the problem, and it is worth noting that Geoff did not have his engine management light in the instrument cluster lit as I had experienced, making the diagnoses of the fault even more difficult.
I suggested to him that the problem might be the Air mass sensor and he duly sent it off to 'ECU' who tested the unit and repaired it returning it to him within hours of receipt.
Geoff the owner of the car was delighted with the result:- quote:-
Refurbished unit, from ECU Testing, arrived 7:35am Saturday, as promised. Took about 10 minutes to re-fit and reset the windows.
I'm very happy to say that the car is now back to normal!
Total Cost = £265.91 inc return postage and Lofty's discount!

Great another happy MB owner.
So you can see from that experience that problems with your MAFS aren't quite such a disaster as they first appear, providing you do not go down the Mercedes-Benz route.
One thing that is worth bearing in mind, the above company will test your ECU/MAFS before undertaking the repair and will contact you to inform you if it need repairing or not, if the MAFS it is found not to be defective the company will only charge a handling fee plus of course carriage, that works out only a few pounds more that a short diagnostics test with Mercedes Benz workshops and as with Geoff's car the fault may no show, so it is money down the drain!!

My Car is using more fuel now the ECU/MAFS has been repaired WHY?

One point that should be born in mind by all owners with MAFS problems, generally owners will complain of the car being sluggish and having no pulling/acceleration power.
This is invariably caused by the lean/weak mixture which is being provided to the engine controlled by the MAFS/ECU.
IN this instance although the car is sluggish fuel consumption my be reasonable albeit a lean mixture providing more miles per gallon will eventually cause the valves of the engine to burn.
It is therefore wise to get the MAFS sorted or at least tested as soon a you realise there is a problem, remember you cannot always really on the Engine management light showing where the MAFS voltage is delivering a lean or rich fuel mixture to the engine but the car will not run well and power is usually reduced or car stalls due to the mixture being too rich.

There have been a couple of instances of which I'm aware where the owners have noticed an increase in fuel consumption after having had the MAFS repaired, this will however be accompanied by increased performance in the engine and it is running condition should be back to the same standard when new.
If you experience this phenomenon and are not happy with the amount of fuel used relative to miles driven, then there are several options:- 1. Run the car for a tank or two and then remove one of the plugs, if the plug is sooted up the mixture is rich.(too much fuel is being provided to the engine)
2. When the engine is warm, get a third party to cup a white cloth over the tail pipe of the exhaust for a minute or so, increase and decrease the revs over that period, apart from a few black smuts the exhaust gases should be free of soot.
3. Take your car to any MOT test station and ask them to do a gas analysis, if the car is running rich the readouts should indicate this.

If you are still unhappy contact the company who repaired your MAFS pointing out the problem they will be able to test your ECU/MAFS for you and confirm that the settings are correct or as unlikely as it may seem, reset them for you.
However of the dozens of ECU/MAFS I aware of that have been repaired I have only heard of complaints from three owners, so it is not a common problem.
Also please read this paragraph.
Hi there Bert,

We can re-test P's MAF on our flow bench if he wants to send it back in, we can compare his MAF readings against a new unit that we use as a reference. I have not known it before but If we have set his MAF incorrectly then we can easily rectify this on the bench. We have also had reports of higher fuel consumption from a couple of customers in the past but when we have tested the MAF voltage it has been perfect and when tested on the gas analyser it has been fine - I am not sure if it is the odd customer that is used to driving with a low voltage MAFS that causes sluggish performance but fantastic fuel consumption as it makes then engine run very weak, but then after having the MAFS rebuilt they have all there power back and tend to use more fuel due to the extra power under there right foot.
We have tried to lower the MAF voltage slightly on one customers MAF before on request in an attempt to improve the fuel consumption but it just makes the engine sluggish again.
Kind regards

I feel sure this mail clears up any doubts you may have about this matter.

If having had your ECU/MAFS repaired /calibrated, and still have the engine management light on then it may well be that one or both of the sensors need replacing.

This up until this week was another £300+ outlay however Tony purchased his replacement units from a company and simply replaced the Bosch connector from the old units , that solved his problem for less than a third of the cost of Mercedes Parts. For more details see page 34.

Be aware the that there are more and more sellers on E-Bay trying to sell ECU/MAFS from 'A' Class cars and making all sorts of claims.
They will not work on your car no matter how much , or little you pay

New buyers of USED 'A' Class cars I would recommend to any new buyer of an 'A' Class to ask for a Full diagnostic printout on the prospective car. It will confirm that there are no outstanding faults, also ask for stored faults to be cleared from the ECM before delivery. Some MB outlets will kick & struggle but insist or walk away.

Let me try and explain.
What you are looking at in this photo.
It is my original unit repaired, you can see one of the seals attached to the ECU, these must not be broken or you will invalidate your two year warrantee.
To obtain this photo I have merely removed the sleeve from the air inlet.
The large tunnel shaped feature is the plastic hood which goes over the two fine glass reeds, forming a wind tunnel, which directs the air entering the system via the Air Filter across the reeds, the two smaller tunnels are formed by to sets on metal legs one behind the other the glass reeds are attached at the top of the legs.
The combined assembly forms the air mass sensor along with of course the electronics shown on the PCB (printed circuit board) above, which determines the fuel/air mixture that you engine receives, both reeds must be intact and not car boned up which is likely to occur if you do not use the standard dry air filter as supplied and fitted by Mercedes-Benz.

The finding on my spare ECU :-
The original ECU/MAFS which was removed from my car when it went defective had been sitting in a box in the garage for just on three years when I decided I would get it repaired I sent the unit to BBA Reman who repaired and returned it. The repair cost at that time 27/07/09 £235.00 plus VAT plus + Carriage, and when I received it back it went back on the shelf for the day when I needed it or I sold the car in which case I will pass it on to the new owner.

However approx 6 weeks ago I received a phone call from a distraught owner who had suffered a failure of his ECU which had been previously repaired by BBA Reman.
He informed me that he was clearly of the opinion that water had some how got in through the hole drilled in the casing of the unit as part of the BBA Reman repair procedure!
Apart from telephoning BBA Reman management, I decided to get my spare unit off the shelf and inspect it.
To my dismay my unit also had an unsealed hole in the top edge which could easily have allowed water to enter the ECU.
Had I fitted this unit and then pressure washed my engine bay or used a steam cleaner, my ECU to could have suffered from water/moisture damage, it fact it is worse than that it kills the ECU Dead, and it certainly will not allow you car to start!
Up to this point I was totally unaware that such a hole was being drilled as part of the repair procedure and in my opinion it is very location negates the seals integral of the unit.

despite the fact that my repaired unit was still just under the two year warrantee I decided to carry out a detailed inspection of the unit and knowingly broke the seals

My further detailed inspection found that a 'pot' had been installed as part of the repair, with wires gong to the PCB, again I was not aw
are or made aware of this by BBA Reman also on close inspection one of the reeds was found to have been very poorly fitted, so much so that again in my opinion it could have affected the performance of the repaired MAFS or indeed its life span, also waste solder particles were left attached to the PCB, not a good idea with hundreds of small sensitive electrical components in the near vicinity .

I have contacted BBA Management about these problems and will be sending them photographs I have taken to substantiate my claims.

If your ECU /Mafs is working correctly as well as your car has a clean air filter then your MOT emissions report should have readings that appear with the tolerances shown on this report This report was done with the EcotecCP23 removed from my car. see also page 93.

I'm very disappointed at my findings and I trust that any owner who has followed my previous advise and sent their ECU/MAFS for repair to BBA Reman has received their unit back in better condition than I had
And please note, if you did get correspondence on how to adjust the 'pot' screw and you found it necessary to do so to get your car to run satisfactorily following the repair, then you would be best advised to check your plugs and make sure the electrodes are a brown colour rather than light grey/white or sooty black. If they are sooty black after a run of a few miles then there is every chance your car will fail the MOT as it is running rich i.e. burning more fuel than necessary, and you also stand a good chance of ruining the catalytic converter.

If on the other hand the electrodes are pale grey/white then the engine is receiving a weak fuel mixture and there is every chance that you will burn out the inlet valves in the longer term.

I'm not going to suggest you tamper with the screw fitted by BBA Reman to correct these problems, you may even make the situation worse. Even members of the BBA management have said that adjustment should be undertaken by a qualified person using specialist equipment, ideally your ECU/MAFS needs sending to a company of you choosing, who should test, inspect, and recalibrate your ECU/MAFS ensuring it meets the recommended standards for the fuel mixture, fuel to air for your engine.
This photograph is self explanatory. a very poorly fitted but new reed

I'm very disillusioned at what I have discovered and sincerely hope that other owners who have followed my advise have not discovered problems of this nature.
My reasons (1) I wasn't told about the hole in the casing
(2)nor about the adjustment feature added
(3)I now discover from a photo taken when the unit was returned and one taken more recently that one of the reeds was very poorly fitted along with waste solder left attached to the PCB

New information.
Also, what I wasn't aware of is, (no fault of BBA as I didn't ask) that even if I had replaced the repaired ECU on my original car at a later date I would have been most fortunate if it had worked. So in effect it was a total waste of money getting it repaired earlier
So if you have a spare ECU do not bother getting it repaired as it next to useless.
Unless the ECU Portion of you present unit goes defective, then you can at cost, get the coded chip removed from you present ECU and have it transferred to your spare ECU. In this case both your spare and ECU from your car would need to be sent to the company carrying out the swap.

As a point of interest electronic defects on the ECU portion of the combined monolith are thank goodness few and far between. However mechanical damage in the form of corrosion is being caused by water reaching the PCB and fine components on repaired ECU/MAFS .

Words of Warning 17/06/09 and in conclusion

If you have had your MAFS repaired by BBA Reman in recent years you need to check to see if the small 1/16" hole drilled in the top, back edge of the casing which is drilled as part of their repair procedure is sealed.

Left unsealed and you are likely to expose the ECU to possible water/moisture damage which will write off the ECU.
If you had to adjust that screw (Pot)to get your car to run then get your fuel/air mixture checked.
Having been alerted to the feature by an owner whose ECU failed following driving through a very severe storm, 'His words not mine' I inspected my own BBA repaired ECU (spare unit) which has been in a box since repair, just two years ago, I was unaware of this hole or it is purpose until this week.
Had I fitted the ECU it could possibly have been exposed to elements not compatible which delicate electronics the likes of which are installed in the ECU/MAFS monolith. The drilling of this hole without it being re-sealed in my opinion negates the very good seals built into and integral of the unit casing and exposes the ECU components to possible water damage, also the two stainless steel clips must be in place around the plug connections if the ECU is to remain water tight.

If you are still intending to use the services of BBA Reman for your repair:- Please be aware if
following repair and refitting of the ECU/MAFS your engine does not run correctly, then you will need the expert help of a garage with the correct equipment to set the fuel/air flow mixture correctly, adjusting the (POT) screw installed by BBA Reman which forms part of their repair procedure. Without checking the fuel/air flow being delivered to your engine on suitable diagnostics equipment your car may run rich, thus failing the MOT on emissions and the car burning more fuel than it should, or indeed the car running weak, thus you will have less power than you should and you stand a good chance of burning out your inlet valves in the longer term.
For this adjustment to be done professionally as BBA Management have indicated to me it will of course cost you extra money on top of what you have already paid for your MAFS repair.
Further more the document sent out with the repaired ECU's by BBA of which I have a copy does not spell out clearly what adjusting the screw does, or that the adjustment should be undertaken by a professional.

Take your car to Mercedes-Benz and they will in all probability refuse to undertake the work at all, as you ECU/MAFS specification will have been altered by the modifications undertaken by BBA Reman, they will simply say 'We will put your car on the 'star' diagnostics" min cost £50.00 and when they get the results simply tell you the MAFS is defective and you require a new unit! £1400.
The chances of them undertaking a manual adjustment of features added to your ECU/MAFS by any Company is in my opinion nil.
So please think before making a decision where to send your ECU for repair

I'm not saying do not use BBA Reman, that decision is up to you BUT simply be aware of the possible consequences of doing so, and if having had the repair done your car does not run correctly, get back to them, to not do so leaves them thinking your car is running OK.

The above information relates in general to the earlier 'A' class and early face lift versions,2002/2/3

However on later W168 models with engines numbers:-
166.940 131
166.960 033/131/133.
166.990 132
166.995 135 These numbers form part of your VIN number
Cost of these units, one owner has been quoted £810 + vat=.£931.50 @15% vat £(51.75@17.5 vat 23/10/09( bearing in mind the information I was given below shop around as the price appears to be vastly different) at the above price they are not a lot cheaper than the other ECU version which is fitted on most A Class in the UK, ECU testing have indicated they are getting one or two a week at present. The ECM (Engine control Monolith )is incorporated with the throttle body with the wiring loom connectors being fitted at the rear of the unit.
Therefore the recognised cleaning method for the earlier MAFS as outlined on the SVC global site is not appropriate in this case, however if you are having problems, spraying carburetor cleaner into the throat of the throttle body will ensure that the butter-fly moves easily and is free of contamination/oil deposits.

ECU Testing are still able to repair these units however they are not able to undertake a full test of the ECU/MAFS as with the earlier version seen above.
For the removal procedure of this unit click on the more info link above.
It appears that more and more of these units are being located on the 'A' class sold in the UK.

However it not all bad new if you have this type of unit fitted on your car, I'm informed by my Mercedes Independent parts dept that this unit cost £500 + rather than the £1300 + for the standard unit and that there is no hot film air mass sensor in these units, that in itself has got to be an advantage, I understand that one of the major problems that arises is the tick-over (idle) which is not possible when the unit goes defective, If I learn more then I will add the information to this page.

Cleaning your engine Bay

You are advised not to steam clean or wash under the bonnet with water/high pressure water or steam cleaning appliances, Both the ECU/MAFS along with the power steering pump and ESP/ABS systems are fitted with masses of sensitive electrical components many of which could be affected by such action.
This PCB (Printed Circuit Board)is the main board removed from an 'A' class ECU this week 10/07/09. It is a total write off, as a direct result of water damage.
This is the second ECU written off this month of which I'm aware. So please head the warning:-
1. Inspect your unit if it is been repaired at some point,
2. Do not wash or steam clean your engine bay.
the cost to you could well be:- unless you can get a company such as ECU testing to switch the information chip from your ECU to another unit, you will have no alternative but to purchase from Mercedes-Benz, and do not be shocked when the bill comes to close on £1400.00 YOU HAVE NOW BEEN WARNED as soon as I was made aware of and discovered a problem, the rest is up to you.

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