My check engine light has been on for about 9 months and the mechanic said it was a oxygen sensor.I'v'e done nothing to the car and the light is out now. Funny and lucky for me. Any Ideas?

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I had a starter motor replaced and since occasionally the battery light would come on but even now that seems to have fixed itself. I have cleaned the battery posts but that didn't help the battery light from coming on and off once or twice a week. Sounds like a bad ground I know but both problems cured? MMMM