My check engine light goes on and off intermittenly, my mechanic says as long as

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all systems are functioning properly it is safe to drive it. This has been occurring for over a year, and I still get 31 pmg..when I put a plastic bag over the gas cap, sometimes that solves the problem. Just the other nite I was driving in San Diego and it went off and the next day it came back on. All systems seem to be working well..thanks, poppycoco