On my TJ I had a CEL once after a weekend of wheelin and I was able to pull the code (it was an O2 sensor fault) but after a few days it went off and hasn't come back (yet). That was about 15,000 miles ago, and my Jeep was just out of warranty. I had another CEL about 3 years ago while it was still under warranty with the same fault (O2 sensor) and I took it to the dealership and they did replace a sensor.