Forgive me if this is in the wrong forum, apologies if it is.

I put my V reg mondeo ghia X auto which has done 79k in for a mock MOT yesterday (I am a little paranoid about my car as my last one died on me doing over a ton on the autobahn in germany and don't understand how they work) and was told it had failed the fast idle emissions on CO levels which were at 0.537% and the second at 0.632% which are both over the 0.300% the lamda results were increased at the second result but it passed the lamda test.I spoke to a Ford garge who said they would diagnose it at an extortionate cost and said becaus of the lamda results my CC is probably gone and I would need a replacement at some £600+!

I was adviced from the garage that did the test that if I replaced my air filter (which was replaced on my last service at 74K) my emissions might be lowered enough to pass. I did this but got the wrong one and took it to a seperate garage who cleaned my air filter and tested my fast idle again and the CO result was 0.000% but my lamda result had increased again which was a rise from before?! Does this mean my CC is going or I have a dodgy sensor somewhere which might be playing up as a drop from 0.632 to 0.00 seems steep for just a bit of dirt in a filter and the garage just told me it happens sometimes which has me confused, could I need a new cat?!

It could mean you need a new Cat but it could also be a faulty lamda (oxygen) sensor near the cat thats giving the ECU a bad oxygen reading so its adjusting the settings to try and get the emissions safe. The only real way of telling is getting a diagnostic done.

Engine being cold would give higher co2 readings than when it's all warmed up...

i had a car in at my old garage with same prob and i put some cat clean in it and gave it a good run to get it nice and hot and it went straight thru the test give it a try its worth it only 8 quid for a bottle i think:)