Thanks so much DavidDavidS said:The O2 sensors measure how much free oxygene there is in the exhausts.
There are two O2 sensors befor the catalytic converters and two after.
The first pair is used to regulate the amount of fuel injected to match the amount of air drawn into the engine. The air-to-fuel ratio is tremendously important to get good performance, and to get low toxic emissions.
The second pair of O2 sensors is used to check how well the catalytic converters work.
Usually only the first pair of sensors wear out. I would not replace the O2 sensors unless there are clear indications those are faulty.
Is there any symptoms? (I have read your post about replacing MAFs next week, so I´d suggest to wait with the decision to replace O2 sensors until after that.)
If the O2 sensors are bad you will get poor emissions values, especially the Lambda value will be off from its ideal 1.00 . (The Lambda value tells how close to ideal combustion the exhausts are after the catalytic converter.)
If you get bad emissions values, first check the function of the MAFs. These measure how much air is drawn into the engine. If the MAFs are not working well, the exhaust values will be off no matter how good the O2 sensors and the catalytic converter work.
If the Lambda value is above 1.00 suspect and check the MAFs and the fuel pressure before you suspect the O2 sensors and catalytic converter.
How to test the MAFs:
jordantii said:Also BMW recomends the change at 100K.
You can also put the car on the DIS and look directly at the 02 sensors and see how they cycle.