check engine light on 1991 Mitsubishi Galant

Why does "the check engine" light come on but no codes come up with a diagnostic check. Light was shut off but came back on within minutes of running. Car has 120,000 mi.

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2 answers 1 comment
Make sure your gas cap is on tight and makes a good seal. We've experienced this two times and both times it was corrected by tightening the cap better.
If the idle speed is too low this can happen, does the light go out if you rev the engine up?
No, it just stays on all of the time.