Check engine light

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Check engine light came on during a long drive 5 weeks ago - was hot and AC was running on high. Shut AC and amp needle clibed noticeably. After resting the car for about 2 hours, the check engine light came back on at start up, but shut off 20 minutes into driving (AC was on and amp needle was low). Next day and for several days light stayed off. Then had an oil change and light has been on continuously for two weeks.