I just replaced the cat and the whole exhaust system. Now there's a black buildup on the inside of the tailpipe. That means its running rich right? My guess is the O2 sensor is bad. How do you tell? It runs great, lots of power, no smoke or any problems. Thanks.

 

You probably have a bad O2 sensor like you think. That's what mine was and my gas milage suffered due to it.

 

Thanks Bumpin'.

Does anyone know how to tell if the O2 Sensor is bad?

 

It kinda sucks to test it cause you gotta probe it with a multimeter when hot (careful!).
Check the values you get with a good service manual to see if the sensor is still good.
If no, then replace......or you can just replace and see if it helps.

 

My guess is that your O2 sensor is old anyways. So just go ahead and replace it.

 

Thanks guys. I think I will probably just replace it sometime.

 

If it's never been done, just replace it. If you're the curious type, trace the wire from the sensor, through the floor and under the seat. Separate the plug there and test the sensor side - the two terminals away from the plastic snaplug that holds the wire to the body should have a resistance of 5.1 - 6.3 ohms at 20degC (68degF). There's another test, but it's way too detailed to post here and probably requires two people.