Emissions failure with new cat and lambda sensor


I had a new catalitic converter and coil fitted last spring and my car has been superb since. However, my car has just failed its MOT on emissions, after 4 repeat tests by the local MOT garage. I’m not technical but here’s was the report says:

"Exhaust emissions carbon monoxide content at idle excessive (7.3.D.2c)

Exhaust emissions Lambda reading after 2nd fast ideal outside specificed limits (7.3.D.2b).

This garage said that it could only be either the cat or the lambda sensor (which they said looked like it hadn’t been replaced when the cat was done) and they recommended I take my car back to the original cat fitter garage to source the problem and before the MOT expires. I did so and this garage then fitted a new lamba sensor, which reduced the emissions problem much but not totally because the emissions are still currently just outside of the MOT requirements ie. will fail the MOT again. I then gave the car a good run for about 6 hours (with this new lamda in place) and am about to take my car back to this garage again.

Has anyone had a similar experience and / or any suggestions as to what the cause must be?

Many thanks, as always


There are a number of scenario from, it is possible that because the lambda sensor has failled that the mixture in the engine has been too lean and the cat has been damage, to another simpler problem like engine oil too old or end of life plugs and leads. Or even the new sensor is out of spec. It could even just be your cat has fouled up with soot and needs a clean out. You can get fuel aditives and you could give it a very good blast. I would be tempted to go for the later myself first.

I would ask the MOT station for the actual readings (for example is the Lamda now @ 1) and if Lambda is OK but enissions out I would go for a replacement cat from the original supplier. 

Hi John

Thank you very much for your help - you’ve been proven right. I forgot to add in my previous post that the cat fitter garage put a fuel additive in and before I then gave the car its long run that day. Today, I took my car back to the local MOT garage first, post the really good blasting that I gave it across Kent & Sussex, and all was fine.

I’m so pleased that I had the sense to join this Club! Its now easy being an MX5 lover and not being technical - but everyone is always so extremely helpful - and right!

Once again, thank you.

Julia (with emissions all properly in tact!)

ps: As I’ve had excellent service from both of these two garages, I shall post their contacts in the general section here in case it helps anyone else. :slight_smile:


Just to point out that a failed/failing precat lambda usually increases full consumption by 10 - 25%.  This extra fuel burn is caused by the ECU trying to balance the lagged signal from the lambda.  The car puts the extra out as unburnt biproduct ie. hydrocarbons(soot).  Over time this soot will overwhelm the catalytic converter to the point that it will not heat up enough to burn off the soot layer and will need replacing along with the faulty precat lambda.

Luckily in your case, Julia, the soot buildup was temporary and a new lambda combined with the run have burnt off the soot buildup in the cat that would have caused the emissions failure legacy from the earlier lambda fault.