Occasionally, my gas cap is screwed on tight enough. The check engine light comes on, usually when the gas tank is just above half. I unscrew it and screw it back on. Eight to ten engine starts later, the check engine light goes out. This time the light was on during a maintenance stop at the local repair shop. The "computer" says O2 sensor malfunctioning and I need to replace it. Gas mileage is about the same as usual. Car sounds fine. The shop says the code indicates an intermittent problem and needs replacing. It was replaced in 2008. I think it's just the gas cap. Thoughts? 2004 Vibe, about 95000 miles.