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air pockets in the cooling system, start by bleeding the lines, plus there is a bypass for the heater core when you have the a/c turned on, or your heater isn't on, this fluctuate between makes and models. but i would go to YouTube and look up how to bleed coolant lines, and that should make it easy for you.

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Q: What causes heater not to get hot in 1999 gmc truck?
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