I was having a big issue with O2 sensors and my car running extremely rich, it is a 99 Mustang GT. The check engine has been on for years off and on, I now have a built motor and the light is back, plugged my SCT in and it tells me 4 different codes, all with O2 and emissions valves, further research today revealed that the #8 fuse in the central junction is linked to all 4 codes. Sure enough, I pulled the fuse and it was blown! Replaced it today and the car was running completely different, no more horrible fumes, smooth idle, great exhaust note (thanks to the cams!). It was like everything finally started to flow smoothly. Then after a little drive, blew the fuse again, replaced and blew again! I believe that this is an issue with either a ground or a short somewhere, any ideas on where to start looking? Just want my baby to run like it should for the first time since the new engine build. Have had the issue before the new engine as well. Engine is roughly 1 year old, been having issue for roughly 3-4 years now.

 

First item is to get a complete wiring diagram for your car. Many shop manuals have this in the back. Next would be to trace each wire that is associated with the blown fuse. Check continuity to ground with each. You might have one wire that is moving as you drive and then contacting metal. Worse case is you can't find the frayed wire and will have to run new wires to each item associated with that fuse. Oh, don't forget to disconnect your battery before tracing.