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Q:

check engine light on 2004 Chevrolet Colorado

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My check engine light comes on and stays on even when the truck is turned off. Overnight it drains my battery and i have to jump it to get it running. Any idea as to what is causing this?
(3) Answers
I had it happen in mine too because of the gas cap. To reset the check engine light, as soon as you turn the key,pump the brakes hard 3 times. That fixed mine. Though it always reappears.
It sounds like your Ignition switch is defective and not completely turning off.

here is some info about the light when it is on while driving

http://repairpal.com/check-engine-light
your alternator might be shorted out internally. try taking it out and having it tested. my truck had the same problems. i also had problems with power locks and windows when the battery went dead