This check engine light has been lighting up the interior of the car for about a year now. Replaced the downstream O2 sensor only to find out that it was the upstream sensor that was bad. Of course the upstream is the hardest to get to. Then the proverbial "gas cap is not tight enough" happens often. When we fill up, it somehow creates a tank vacuum and it takes quite a few cranks to start the car back up. Once it starts and runs for a few minutes, its fine. But the CE light is still on. I tried the disconnect the negative battery terminal, reconnect to reset and drive continuously for 50 miles. It worked! The light went off............for about 3 days and then of course came back on again. With over 100,000 miles, I'm not sure it's worth the code scan. The day I fix that upstream O2 sensor, the timing belt will disintegrate. Always, always, always. Too bad too, cause it actally runs pretty good considering. At least for a Pontiac.