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Q: Heat issues

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When in idle for extended periods of time, the vehicles temperature goes up, once moving again the temperature goes back to normal. when heat is on, temperature fluctuates as well. Heat works for first few minutes of being on then stops working, this is when the vehicle temp will rise. What is the most likely cause?

My car has 140000 miles.
My car has an automatic transmission.

The first thing to check why it runs hotter at idle is to see if the coolant is full and there are no leaks. You may need to have a cooling system pressure test if system is low on coolant to see if the radiator or water pump is leaking. You may need a mechanic like one from YourMechanic to do a test on the cooling fan clutch to see if it is locking the fan to turn faster when the cooling system starts heating up. If system is full of coolant and fan clutch is working properly then the thermostat should be replaced for not maintaining a proper temperature.

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