I've got a vehicle that could use a rering job but still takes me to and from work almost everyday without much problems. I know in due time I'll have to tear into it but it's still very road worthy.I've removed the O2 as well as the plugs just to see if any fouling has occured. The O2 looks normal and the plugs are alittle on the darker side but not bad IMO.
My question is what does the O2 read as an engine ages. Does it read it as richer or leaner. I'm just wondering if ECM compenstate if any? I've had a slight drop in mpg.

 

They read the amount of oxygen in the exhaust and the ECM then adjust the injector pulse width to get the mixture that it is preprogrammed to run. Even with some oil burning, they will do the same thing, though over time may become fouled to a point where the don't read as accurate which can either give a lean or rich condition.