Where is the Rear O2 sensor located? What kinds of things will upset this? I have a WSP Y pipe that has been on there for about 15k miles a carsound cat that's been on since the Y pipe and a Greddy SP that has been on for about 3 weeks and about 1000 miles. Recently I added a CAI, and since then I've kept tripping the CEL. This is the most recent of CEL's. It doesn't trip immediately, I can drive for 10 minutes before it trips and then I can stop and reset the ECU and drive for an hour with no errors.

Please help