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Q: When heat is on temperature gauge goes towards cold. When heat is off it goes back to being in the middle. 2001 Buick Regal

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When I put on the heat when the engine is warmed up the temperature gauge goes from in the middle of cold and hot , down one quarter towards cold. When I turn off the heat it slowly rises back to being in the middle of cold and hot.I took the car to Firestone 4 months ago to change the coolant and I remember seeing red or pink fluid. This car takes Dexcool which I'm told is orange. Is it possible the shop put in Toyota fluid which I've been told is red and if so am I making the situation worse? Do I need to have the fluid flushed again? Do I have air in the radiator? Do I need a thermostat? Thank you.

My car has 63834 miles.
My car has an automatic transmission.

Hi there - the temperature gauge behavior you describe would indicate that you need a new thermostat. The thermostat should keep engine temperature within a close range - close to the center of the gauge from a visual point of view. The fact that turning on the heater allows the engine to cool down indicates that the thermostat is not doing it’s job. It’s impossible to know what the service center put in your cooling system prior, but you are correct - Dexcool, the GM-recommended coolant, has an orange tint to it, and should be used in your engine. I would recommend having your thermostat replaced by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, replace your thermostat, refill your cooling system, and bleed it. Your temperature gauge should remain centered from then on, and you can be sure the correct coolant will then be in your engine.

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