99 Blazer LS Service Engine Light only when....

I'm on the highway going up hills. Everytime I drive my truck up a hill on the high way the service engine light comes on. Driving on a flat area doesn't make the light come on. So when i finally get to where i'm going and
pull over, rev the engine for 10 or 15 seconds the light goes off. What is this telling me or what could be going wrong? I don't hear/see or feel any thing unusal when this happens. I tried driving it in the city going up hills and such, but it seems to be working perfectly fine even when the light is on. Anyway, should i be concerned about this or what? Any Help will be greatful. Thanks
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I would guess it's a knock sensor problem....(or maybe a knock causing said problem). But remember .... a "guess".
~KJ/TLGM

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another thing i noticed is the light comes on when the engine/tranny changes from 4th gear to 3rd gear going up the hill. that's when the light starts flashing. I'll take it to autozone or what have you and see what the computer says. Right now the light is off, will they still be able to read it from 3 weeks ago. it's been going on for 3 weeks now. Thanks for all the info. i'll do that tomorrow and see what i come up with.

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it happend yesterday morning so i'll run in today and see what's up and get back to ya.

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nothing came back on the computer and i hanv't been on the highway to get the light to come on. but when it does i'll get the info and repost it on here again. Thank you for your help

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