I've had rich fuel trims for a few weeks now, and engine code P0172 has been recurrent. for the first while my LTFT's were at -22.6, I found a burned spot on the o2 sensor wire harness, but no exposed wires. I put some heat shrink on the burned spot and my LTFT's went down to about -12.6, sometimes the fuel trims come to normal range. the code is still there. I'm wondering if my o2 sensor is fouled, and if my catalytic converter is plugged, or the rear o2 sensor has gone bad.