Thread: Bad O2 Sendor codes - solved!

I had posted about this earlier that my DIL's 2004 Grand Prix was throwing codes indicating bad O2 sensor (initially just the Upstream sensor codes indicating bad sensor and bad sensor heater circuit). I replaced the upstream sensor but the problem persisted and in fact now both sensor heater circuits were showing bad. Her uncle took it to his "expert" mechanic who changed the sensor again and supposedly checked the wiring - after a couple weeks, he could not fix the problem and was talking about all kinds of things like replacing the MAF or the Cat, etc. Well, I got the car back and started doing some in-depth troubleshooting. What I discovered was that there was no power getting to the heater circuit at the female plug on the harness that the O2 sensor plugs in to. The fuse was good, so I started checking the wire harness closely. I found a spot un the underside of the harness where apparently the harness had rubbed against the serpentine belt and the belt had cut through the harness and literally cut the wire from the fuse box to the O2 heater. I soldered in a splice of wire and checked the continuity and it was all good! FIXED! Or so I thought! I was still not getting 12v to the O2 sensor plug. The fuse was good, so what was going on? Well, I pulled the fuse and stuck a paper clip into the hot side of the fuse receptor and was getting 12v as I should! But, when I plugged in the fuse and measured at the top of the fuse (the little metal point on top the fuse where you can check voltage), there was 0 volts! Ahah! The problem was a worn female contact on the fuse box for the O2 sensor heater circuit! Not wanting to replace the entire fuse box, I devised a jury rig to address the problem. I went to NAPA and bought some little "fuse taps" and shoved a fuse tap into the slot, then plugged in the fuse and there was now sufficient contact to provide 12v at the fuse and hence 12v at the O2 sensor plug! Reset the codes and viola - it's been working fine ever since.

Thanks All. I was pretty happy that I figured it out. Bad fuse box is not a typical problem. Live and learn.

I had that issue with the fuse for my low beam driver side headlight, I just bent 1terminal on the fuse 1 way and the other terminal the opposite way and plugged it back in . 3 years later and 85k miles and it still works lol...