96 XJ, 4.0L, AW4, NP231, 4.5 inch lift. I recently replaced my entire exhaust system including convertor, muffler, tailpipe and rear O2 sensor. It had been giving me a "Convertor Efficiency below threshold" OBDII code. The new exhaust system fixed that. BTW, the O2 sensor is a dealer part, not an aftermarket. The other parts were aftermarket, and the muffler is a Magnaflow (if that makes a difference). We spent yesterday doing some serious wheeling including rocks, sand, and mud. At the end of the day I had another Check Engine Light, and this time the code said that "O2 Sensor no. 2 Voltage High". What does that mean. Did the sensor fail? or could I have broken a wire or filled the connector with mud?