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Q: Heater blows cold air when vehicle is stopped.

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I recently got new head gaskets on my car, along with many other things like thermostat, plugs and wires, alternator. I drove it today after it was sitting in the shop for 2 weeks (I was getting new wires on it), and the heater blows out cold air when at a stop light. I just noticed it today. I got the head gaskets done about a month and a half ago, and didn't have any problems until tonight? My car isn't overheating, it simply just blows out cold air when stopped. It has been very cold here, and I didn't give it much time to warm up, so maybe that has something to do with it? However, I did drive for about 15 miles and it continued to blow cold air at stops.

My car has 134000 miles.
My car has an automatic transmission.

You may have a clogged heater core that may be prohibiting the outer linings of the heater core to get hot. As you know, the heater core is a series of tubes that the coolant runs through that generates the heat from the warm coolant after circulating through the motor. These tubes can occasionally develop a buildup on the inner walls such that when the heater core is "flushed" this buildup on the inner walls of the lines remains and still allows the coolant to flow through the heater core. It is important to note that when the heater core is "flushed" it is possible to get good flow through the heater core while the inside walls of the heater core remain caked with buildup. As a result, the buildup on the inner walls prevents the temperature from rising to its capability due to the insulating effect of this buildup on the inner walls which will prevent the necessary heat transfer to properly heat the inside of the car. I would recommend having an expert from YourMechanic come to your location to diagnose and replace your heater core.

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