I have a 95 z28 that unfortunatly due to the county i live in had to get an emmissions test with my inspection and it failed. It actually passed the high speed emissions results which was at 1688 rpm but failed the low speed emissions which was at 808 rpm. It failed the hc (ppm) pretty bad with a reading of 452 with a upper limit of 220 and it also failed CO% at 1.42 with a upper limit of 1.20. I am sure i need a new cat. converter but the inspector also said i would need a new O2 sensor due to it failing the CO%. The question i have is which O2 sensor should i replace, i dont want to have to c/o all 4 of them. My check engine light is not even on . Thanks for any help you guys could give me. jason hall

 

Your 1995 only has two O2 sensors, one in each branch of the Y-pipe, both before the cats.

High CO and HC can be the result of running rich. I'd look at the PCM with a scanner to see if the long term fuel corrections (BLM's) are elevated. That would indicate a faulty O2 sensor, leaks in the exhaust system before the O2 sensors or misfires, all of which could cause it to run rich.

Try and narrow it down between the two O2 sensors, some other problem, or a weak cat.

 

It didnt sound right having 4 o2 sensors but down at autozone they shown me a picture showing 2 sensors before the cat and 2 after, i really should of jacked it up and looked. ?Thanks for the info. jason

 

They were most likely showing you pictures of a 96-97 car then.

 

Wouldn't my check engine light be on if one of the O2 sensors was not functioning properly? that just seems strange because i have had o2 sensors go bad quite a few times on my other cars and the check engine light is always on.

 

OBD I is dumb. It's wouldn't complain unless the O2 sensors totally died, and them not always. OBD II is smarter and knows the limits the engine should run at and complains if it's not with-in them.