Hello. I have a 1994 Z28 A4 with some mods done by the previous owner. Since the headers and the entire exhaust system are all aftermarket, I was wondering how many O2 sensors came stock on the vehicle. This vehicle only has two located near the end of the headers. I should also mention that the entire exhaust is off road as there are no cats. My check engine light always comes on when the car warms up and I suspect that the oxygen sensors (Bosch) are incompatible with the car as it seems to run rich (black exhaust blowing out the tailpipes at times). How can I get the check engine light to go away and ensure that my car is optimizing the correct fuel/air ratio? Any help is much appreciated. Thanks.

 

you would only have 2 front O2 sensors. If the headers are ceramic coated, that could have killed the O2 sensors if they were not changed after header install.

 

1. Have it scanned so you know what the problem is. Doesn't do any good to guess.

2. The light goes off when you find the problem and fix it.

Many people here have had problems with the Bosch O2 sensors. The OEM AC/Delco sensors seem to work better.

Some possible problems:
-O2 sensors were damaged or contaminated, as noted in the post above.
-exhaust leaks before the O2 sensors are causing a "false" lean condition, which causes the PCM to pour in fuel the engine doesn't need.
-excessive misfires, causing a false lean condition
-if the headers required "extensions" for the O2 sensor harness, the extensions could be faulty (or even home made).