Help me understand my MOT failures...

[Feeble]

Earlier I spoke to the garage where my car is being tested. They told me it had failed on a few things, including emissions. They told me the emissions is probably not something that they would be able to sort out themselves due to the type of car it is (W202 C43), and that I will have to take it to a specialist, which I can do no problem.

I decided to have a look at the results online. I am confused by what I am reading.

Exhaust carbon monoxide content after 2nd fast idle exceeds default limits (8.2.1.2 (b))
Exhaust hydrocarbon content after 2nd fast idle exceeds default limits (8.2.1.2 (b))
Exhaust lambda reading after 2nd fast idle outside specified limits (8.2.1.2 (c))
Exhaust carbon monoxide content at idle exceeds manufacturers specified limits (8.2.1.2 (b))

Does this mean that there are four separate problems with the emissions?

Also, the car has been in the bodyshop for the last month having the inner and outer sills replaced, so the car hasn't had a good run in a while. Do you think that a blast up the motorway would clear the issue?

The car needs a service now anyway so I will get that done at an independent specialist at the same time as the emissions hopefully.

The garage did also tell me that the issue is probably with the catalytic converter and said they would go ahead with pricing a new one up. I'm hoping that is not the case as it's a full MB stainless steel system so would probably be difficult to find, not to mention expensive.

Hope some of you guys can help me understand the issues!

Matt

 

[a111r]

Matt, it's one problem of air/fuel ratio, which could be a failed lambda sensor.

Yes, a thrash down to PCS will help.
A

 

[zipdip]

Well Matt get the car serviced a clean air filter never hurts when it comes to the MOT, also maybe a new set of plugs,as you state your car has been hanging around in a garage,this may well mean your cat needs a clean out,since the new MOT came in last year it is deisels that have been under the cosh for immisions,the advice there is to give the car a very good run and make sure you make the MOT booking and turn up just before and stay with the car and keep it running,you need the exhaust gasses hot when they test it,even though your car is a petrol following the diesel way is the best way to pass,I would suggest you buy a can of Cataclean at Halfords it will clear out the cat.

 

[Teego]

I always take my cars, petrol or diesel for an Italian tune up on the way to the MOT. Last year our diesel SUV failed because I hadn't done that. Fortunately the good guys at the test station took it for a run and passed on retest.

 

[Feeble]

Yes, I am very annoyed with myself that I didn't take it for a run before the MOT. I will call PCS tomorrow and get it booked in for the service and see what they say about the emissions. But I think running up there and back should do the car some good!

Thanks guys!

 

[AJA]

Fresh service with all filters
Catalyst clean
Carbon clean
And a good blast on a 'private road.'

As above, it could be a dodgy Lambda (some call them exhaust gas) sensor. A good garage with decent diagnostics will soon tell you.

 

[andy27168]

Matt,
My 43 failed on exactly the same thing after being in the body shop for 6-8 weeks getting a respray.
As it still had an MOT I used it for the daily commute on the M27 instead of my Audi for a week by the retest 7 days later the emissions passed easily, I was worried as you possibly even more so as I only had two of the four cats still fitted.
As you are getting it serviced anyway get Olly to hook the STAR machine up and check the Lambda sensors are in spec still as it isn't unheard of them to fail, though I am willing to bet using it for a day or two and it will fly through the emissions test.

 

[johnsco]

I always take my diesel for a good Mexican Tune-Up on the way to an MOT test.
Never failed one yet !!

 

[Feeble]

All booked in for a service next week.

Just spoke to the garage again and they've said the Lambda is missing and the socket has been welded over. It did pass the MOT in that state last year though so I am not sure it really makes a difference.

Just hoping the service and run up to Waterlooville sorts it all out! They said they dont take bookings for retests, they just fit it in as and when, so I cant sit there waiting for the retest with the engine running really...

 

[Sidewall]

CO=Carbon Monoxide, is the residue of combustion
HC=Hydrocarbon, this is unburnt fuel
Lambda=oxygen sensor=fuel/air ratio
I'm not familiar with your model, but most cars these days have 2 lambda sensors, one before and one after the "cat" so exhaust pipes have holes for the sensors, some non OEM pipes will have extra holes to allow all sensor positions and options depending on the precise model of car. These are sealed up if not used.
Something as simple as an exhaust leak drawing in air can fool the lambda into thinking the mixture is too lean when it isn't. So the CO goes up because the mixture is now too rich. Because it is too rich the engine can't burn all the fuel so the HC goes up. Because the lambda is sensing extra oxygen the lambda figure is too high.
So the readings are to hell in a hand cart and the only thing wrong is a leaking exhaust or a faulty sensor.
Of course if you are unlucky, it may not be that simple......Good Luck.

 

[grober]

The lambda sensor/s form part of what is known as a closed loop control system for the car's fuel injection system to help fine tune the air fuel ratio and thus the exhaust combustion products. Its explained here in terms of a tumble dryer but the principle is the same
https://www.electronics-tutorials.ws/systems/closed-loop-system.html
ps without the feedback control of lambda sensor/s its unlikely any cat system could meet modern emisssion standards

pps Quite frequently the fuel injection system operates in open loop mode during cold starts but switches to closed loop on system warm up-- that's why there are strict criteria in terms of engine temperature to be fullfilled before emissions are measured for the MOT test
part nos
EXHAUST SYSTEM USED ON EIGHT-CYLINDER GASOLINE VEHICLES MERCEDES C-KLASSE [Car] [CHASSIS] (EUROPA)

 

[esscee]

Maybe the moral of this thread, make sure you know when an MOT is due and service the vehicle beforehand and then enroute to MOT station give it a good 30 minutes operation to ensure the vehicle's systems are operating correctly.

 

[zipdip]

Well if I was the op I would find another MOT garage,use one thats takes bookings ,the new tests mean you meed to be there,especially if you have a diesel,ok you have petrol,but maybe after the service the car will pass no matter what.

 

[Feeble]

Well if I was the op I would find another MOT garage,use one thats takes bookings ,the new tests mean you meed to be there,especially if you have a diesel,ok you have petrol,but maybe after the service the car will pass no matter what.

I got some Cataclean off the back of your advice as well. Would you put it in before my 10 mile run for its service, or after it is serviced and 10 miles before my MOT?

 

[400ixl]

Well if I was the op I would find another MOT garage,use one thats takes bookings ,the new tests mean you meed to be there,especially if you have a diesel,ok you have petrol,but maybe after the service the car will pass no matter what.

Why do you need to be there in person for the new MOT test? You are not allowed to engage with MOT once the test has started so it makes no difference. They should only perform the emissions tests once the right conditions have been met.

Any car which is running as it should would not need any intervention. We have a car here which has been bodged by someone and needs putting right and then testing. I would hazard a guess someone previously had decatted it and run a lambda emulator on it. It may well still be in that condition.

 

[zipdip]

Well of course you can park your diesel outside the MOT garage and go to work or go shopping,but the fact is the exhaust gasses need to be hot to give the best chance of your car passing,of course it is up to the owner,in this case we have a petrol car left hanging about then put into a MOT garage that does them in between more important matters,and it has failed,I still say pick a garage that has bookings and be there.

 

[zipdip]

Put the cleaner in after the service,best of luck.

 

[Jamiewatkins2004]

A friend of mine had a very old golf with loads of miles on it, he put a fuel additive in the fuel so it burned easier and passed emmissions tests until it finally gave up.
He also serviced it with fresh filters and oil as well prior to test.

Sent from my SM-G960F using Tapatalk

 

[Feeble]

The car has just gone straight through its retest.

I had the Lambda sensor changed at PCS as it was not an OEM part, and an A service. Didn't use the Cataclean in the end.

Also learned that the car does have a decat system, with only two of the four original cats in place. The guy I bought it from told me it had a decat system but when I saw the remaining cats underneath a few months back I thought he was just saying it. It looks like quite a nice system under there.

Thanks for all your help anyway guys!