My mil light will come on randomly throwing the PCM fault code. It will stay on for a day or 2 and than go away. Iv noticed my mileage has gone to crap. I get 14-15 mpg cruising at 45 mph at 1500 rpm. I used to get 20, well I thought a bad fuel filter was the cause but after replacing that I got nothing. Well today I found my o2 sensor near my #1 cylinder side wasnt working properly. Can a faulty o2 sensor cause the PCM code to be thrown? If I need a new PCM what's the best place to find one?

 

A bad O2 sensor would definitely drag the fuel economy down.
It could also cause the CEL to light but it would really help to know what codes it threw.
If you know for sure the O2 sensor is bad I'd replace it and see if the fuel economy comes up and the CEL stays off before replacing the PCM.

 

It threw the p0601 code. I replaced the o2 sensor that was bad yesterday, but today the light came back on. Could a bad ground do this possibly?

 

The P0601 is a PCM error internal to the PCM meaning you need a new one. I would call around to junk yards to find a new one ask for the same year, engine, trans and maybe even transfer case that you have this will eliminate any questions as to if it will fit/work, the hard part is going to be finding a used HO computer, regular V8's have a different program but you could try it. The P0601 however should not show any issues to you just the MIL.

 

I'll give that a shot. Will the bad PCM affect my mpg's?