My mechanic just replaced my heated rear O2 sensor after pulling a P2097 code. He test drove it twice, then when I picked up the car and drove off the CEL popped back on 10 miles down the road. He wants me to take it to the dealer now for a more complete diagnostic. It's a 2005 with 66K miles and out of warranty - any suggestions on what I might do before taking it to the dealer and racking up a big diagnostic bill? Thanks for any help you can lend!