Mk5 fiesta 1.25 failed MOT on fast idle emissions, CO = 6%, lambda =0.810, HC = 230ppm. Tried changing the air filter, coolant temp sensor, first lambda sensor and fuel rail and its still no different.

Enigne runs fine and economy is still good although smeels really rich when cold, garage thinks it wants a new cat and post-cat lambda sensor, going to try the sensor but dont think the cat is the problem as its well rich anyway.

Anything else I could try changing? plugs and leads looked recent and all the plugs look good. Doesn't seem to have a MAF so struggling off other things to check??

 

I thought the point of the cat was to remove CO2 emissions? Not sure how you can test a cat, other than it's either doing its job or it isn't, based on CO2 output?

 

The purpose of a cat is not to remove CO2 but to convert CO into CO2. CO2 is considered to be less harmful than CO.
Changing the post cat lambda will not change anything. The engine is running far too rich.
Is the exhaust manifold leaking? Then that would explain it.
What signal is the front lambda giving?

 

It went to Ford to be put on their diagnostic machine and they said the front lambda sensor was stuck on 0.8v rather than switching. They suggested trying a genuine ford sensor instead. No sign of the exhaust leaking either.

I fitted a genuine ford sensor and checked myself and from cold the lambda works fine for the first 5 minutes or so, the voltage is switching and the afr is good but then over about 10 secs the switching gets less and less and then the voltage just sticks on 0.8v so the ecu thinks it lean and puts more fuel in.

I have tried 3 or 4 sensors, two of which have come from other working cars and all do the same thing, I have an LC-1 wideband sensor so have monitored it from the tailpipe and it shows the mixture is fine to start with then when the lambda stops switchng it goes rich. I've no idea what could be causing this?

 

A voltage of 0.8 means the mixture is rich. Ecu should lean the mixture, but doesn't for some reason.
Does this car have a map-sensor of an air mass sensor.
In case of a map-sensor it could be an air leak.

 

I cant remember now how many lambda sensors this car has (my wifes old car was a 1.25lx)
But if i remember correctly i had exactly the same problem and was fixed by changing the lambda sensor, now this has me thinking there is two and it was the second one that was changed, in your words the "post cat sensor"

 

It doesn't have a maf sensor, the guy from Ford assured me it did but I can find one and theres no mention of it on microcat, cant find any evidence of a map sensor either.... (Its the later model 1.25 with the airbox over the inlet manifold)

Yea it does have 2 sensors, I though the same but the ford diagnostic guy told me it was working fine and that its not actually used for fueling only for ecu diagnostic to check the cats working. I have a couple of spare sensors so I'll give it a try swapping it anyway.