Originally Posted by McGuinn
CO high and lambda low (low oxygen)...
A low voltage indicates a high oxygen content and a lean mixture...
That means that you are not getting the right measure of air into the system (too little)...
It also means the following:
"Carbon monoxide is emitted by cars, because the temperature is too low to effect complete oxidation of the CO to CO2, because the time (i.e., the residence time) available in the combustion chamber is too short, or because there is insufficient oxygen present. Usually, it is more difficult to design and operate a combustor for very low CO than for very low unburned hydrocarbons. "
From the MOT results, we also know that the HC is high. This tells me that there is an engine fault brought on by the AFM being faulty...
That's my final guess...
OK, I am not familiar with the 146 in detail. But here is my 2 cents.
The Lambda reading is consistent with the high CO reading.
Since the problem didn't change with the O2 sensor change it may well not have been faulty!
With the low Lambda reading the O2 sensor should be sending a high voltage(0.8V - !.0V) reading to the ecu.
Even with a faulty Cat or O2 sensor the CO should be less than 5-6%.
Having changed the thermostat, which WAS faulty, there was no improvement. This leads me to the engine coolant temperature sensor(CTS). If it is faulty or has a corroded connection the ecu could be receiving a cold engine reading, causing a rich mixture. However, you are not getting CTS fault code apparently!
Check the resistance of CTS, it should read 2k to 6k ohms cold and around 300 ohms hot. The precise reading is not important.
Check also for corrosion under the plastic sleeve behind the terminal on the CTS wire and or broken wire.
Failing that, a long shot would be leakage from a faulty fuel pressure diaphram into the inlet manifold.