My check engine light is on and I'm pretty sure its the O2 sensor because a
mechanic told me it would need to be replaced soon. Planning on doing it myself
but I'm confused on whether there is 1 or 2. All the parts sights say 2, but all the
service manuals only give the indication that there is 1. Have 328i, '97.
OBDII does have 4 O2 sensors. The 2 pre cat sensors are regulating while the 2 post cat sensors basically analyze the emissions to make sure the pre cat sensors are doing their job. This is for US versions at least, to cut back on smog levels and make the cars more "environment friendly".
Originally posted by bigred318@Jan 12 2005, 11:44 AM so which one do you have to replace when your 02 sensor goes out?
Generally if you pull codes, it indicates which bank, and whether it's before or after the cat. Or, you could break out a multimeter and check them individually, or just replace all 4, because if one's gone bad, the rest are usually on their way.
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