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Q: thermostat?

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the temperature gauge goes up to the middle (straight up) once the car heats, then once I turn the fan on for the heat it backs down to the left and stops halfway between 100 and 210. Is this normal or does the thermostat need to be replaced?

My car has 213000 miles.
My car has an automatic transmission.

A: It is not uncommon for the temperature gaug...

It is not uncommon for the temperature gauge to drop when the heater is switched on, especially in colder situations (below 40 degrees f). The thermostat should be able to control the engine temperature in a way that you should be able to still get hot air into the cabin when the heater is engaged. I usually suggest that as long as the heat coming into the cabin is sufficient to keep you warm, I wouldn't normally worry about the lower temperature on the gauge. If the air coming out of the vents isn't getting warm enough though, I would try changing the thermostat. If this is something that still worries you, and you feel that you could use a hand in diagnosing or repairing it, I would suggest consulting a certified mechanic, like those available at YourMechanic.com.

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