Ok, so I did the key on/off trick for my CEL and came up with a code 21. According to the chart, that means o2 sensor voltage doesn't change. Problem is, the code doesn't tell me which one is bad. I get a code 21, and then a 55. 95 Grand, obd1 obviously.

Based on previous experience i'm assuming it's the rear o2...but I wanna be sure.

 

Well the 55 just means that the codes are done. Either way, the o2 sensors are very easy to check and take off. You get a torch and take ur dvom and check the signal wire, i think the white wire but hold the torch close to it until you think its really hot (depending on the heat) and take the torch away it should fluctuate from 1-5 volts. Good luck!!!

 

Oh and sorry, the 21 means the upstrem o2 sensor. Either shorted to voltage, slow response, or slow o2

 

I don't have a torch, but if it's the upstream then i'll just change that I guess. I'm using this chart here:

http://jeephorizons.com/phpbb/viewtopic.php?t=2719

But it doesn't specify which sensor it is.