Took the car in as the cel lite was on.The code was po135.Car has 62,000 mi on it and I believe this was replaced once in warranty.The dealer wanted$331.85 to reoair.I stopped at Autozone and priced the 02 sensors and they said there are 2.1 was $220 and the other $500.Can I check these myself anyway and where would they be located?TIA