My car had the check engine light on for an 02 sensor. It was showing the downstream sensor in bank too. I changed it, and two days later it came on for the first sensor in bank 2. I changed it, it passed emissions for PA, then on the way home the CEL came on again. This time for the first sensor in bank 1. Is there something else going on here, or are they just all wearing out? Each sensor is showing codes such as P1131 and P1151.
