Failed Emissions Test on MOT and its not the JUST the Catalytic Converter - Is My Car a Gonner?

fallen121

Petrol VW Golf 08 Plate. Always sailed through MOTs, regularly serviced, no advisories from last year. This year it fails without warning:

• Exhaust Lambda reading after 2nd fast idle outside specified limits (8.2.1.2 (c))
• Exhaust carbon monoxide content after 2nd fast idle exceeds manufacturer's specified limits (8.2.1.2 (a))

Basically the mixture before it even gets to the cat is way illegal. The cat probably needs replacing too but garage don't understand what is causing this and if they can't solve it even a new cat won't get it through if the mixture is illegal before it even gets there. It's way too high to be solved by changing spark plugs, filters or using engine additives.

No warning of this and the car has been regularly maintained. Not driven as much since lockdown but still taken out at least twice a week.

Anyone any idea what might be causing this? No way I can afford a new car so face losing my wheels if they can't solve this. No warning at all. Car always started fine and ran sweet as a nut. If I am entirely honest sometimes when it started up it sounded a bit like a diesel but garage have spent all day diagnosing and don't know what's wrong or how to fix it.

Alanp

Faulty sensor. ?

AdrianC

The MOT emission test is all done at the tailpipe. There's no way from that to know what's happening before the cat, just what's coming out the end...

Assuming the really obvious, that it's up to temp (but that would be high CO), and there's no fault codes relating to the pre-cat lambda...

...then a dead cat is the most likely cause. One of the jobs of the cat is to oxidise CO into CO2.

What's the actual lambda, CO and HC readings?

fallen121

Alanp said:

Faulty sensor. ?

• No engine warning light on the dashboard
• No jerks when starting
• Fuel consumption not any higher than normal

I would have expected at least one of those if the sensor was faulty...?

fallen121

AdrianC said:

What's the actual lambda, CO and HC readings?

I don't know because that's the printed paperwork you get when the car is returned to you and its still at the garage. I logged on to view the MOT failure online and it doesn't provide that info.

I get what you say about what the MOT covers but I believe the garage have gone beyond the MOT and tested pre cat in an attempt to diagnose the issue. They know I am keen to keep this on the road if possible. And the lambda sensor was mentioned on the phone so presumably they have looked at that.

I think the garage is under pressure with cars coming in all the time. This was expected to be a fast turnaround and hasn't been and with mechanics off left right and centre self-solating they are stretched to the limit and weren't expecting this and haven't allocated any bodies. So as I don't need the car have agreed to leave it there until middle of next week.

Apologies if the info I am providing is second hand and uninformative but garages tend to be very brief on the detail when explaining things to women and in this instance I can't pretend any in depth knowledge, just common sense on basic things to do with how a car operates. 

Nobbie1967

High lambda can be caused by an air leak on the exhaust after the cat. I had this on my car when one of the sleeve clamps to the backbox had started to deteriorate.

fallen121

Nobbie1967 said:

High lambda can be caused by an air leak on the exhaust after the cat. I had this on my car when one of the sleeve clamps to the backbox had started to deteriorate.

That is really interesting. One of the advisories on the MOT report says:

• Central exhaust sleeve clamp corroded

cattom

If as you say it has a corroded exhaust clamp, then it may be drawing in air where it shouldn't. exhaust clamp for that vehicle will not be a fortune. and is a relatively simple fix.