GMC Troubleshooting Questions? Ask a Mechanic for Answers
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Check and see if both heater core hoses are very warm to hot to the touch and about the same temp in feel.If one is cooler and the other is allot warmer this indicates the heater core is restricted needing flushed out or replaced.If they are both hot to the touch then the temp actuator itself is most likely at fault needing replaced.I have had to replace this actuator several times before for this fault here.It will need calibrated as well after it is replaced.
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