I have a 95 SHO. Some years back the plug for the rear O2 sensor melted together. No way to unplug it. Long story why. I got another upper plug and some wires from a junk yard, but I never moved forward on doing much about it. The smog center guy said I'd better not leave that sensor in much longer because that bank is running too rich and it will foul up the catalytic converter. So, I got off my butt and wired the new pigtail in, at the end of the wires of the old O2 sensor. Just cut the old sensor off and used crimp connectors to hook them together. Before the swap, the error code would state things were to lean, which causes that side to try to run rich to compensate. When I wired it up, I got two wires crossed. I think it was a heater line to either the ground or signal line. Of course, I still got a check engine light. After going back on the lift I found the error and corrected it. I drove up and down the street and got no codes. Yay! But, on the way home, I got a light, then it went off, then back on. Eventually it stayed off. The light didn't go off, when it was wired wrong. It just took a couple blocks to finally turn on. I checked the codes and now have a 173, rather than 172 stating it's too rich now. So, the question is... Is the computer trying to figure this all out, or did I mess up my O2 sensor, when it was wired wrong ? Not used to having a check engine light go off, that wasn't due to oil pressure.