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Q: While driving defroster set to hot blows hot air as it should, while idling it is blowing cold air.

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While driving defroster set to hot blows hot air as it should, while idling it is blowing cold air. When I stop at a light the air almost immediately becomes colder, as I accelerate the air begins to get warmer, seems to take longer to switch from cold to hot than it does to switch from hot to cold. There is no change in fan speed. The car never did this before.

My car has 108000 miles.
My car has a manual transmission.

A: First, the coolant level should be checked ...

First, the coolant level should be checked to make sure the radiator is full. While the overflow tank may be full, the radiator cap must be taken off to check the level inside the radiator. If you find the level is very low, you have a coolant leak that must be fixed. If coolant levels are normal, an engine thermostat or heater control problem may be what you're experiencing. Consider hiring an experienced technician like one from YourMechanic who can come out and take a closer look at your lack of heat concern to offer a more personal diagnosis.

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