Q: Heater works intermittently, when not working car overheats and thermostat is new

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The car is overheating. I replaced the thermostat. The fans are working. I have flushed the radiator. The water pump seems to be working. The car runs a normal temp at idle and low speeds. It only gets hot once it's run at 50-60 mph. The heat isn't working consistently though. When the heat works the car will cool down a bit, but as soon as the heat stops working it gets hot immediately. Any tips would be greatly appreciated. I'm good with basic car repairs, but I haven't seen this before.

My car has 261000 miles.
My car has an automatic transmission.

Hi there. In most cases, when you have issues with the heater working and the vehicle overheating, it’s caused by an obstruction of coolant inside the heater core. The cooling system circulates hot coolant / water into the heater core to heat the elements and blow warm air into your vehicle. If there is debris inside the heater core coolant tubes, it will have difficulty completing the circuit through the entire cooling system and cause overheating issues. The best way to know for certain what is causing your overheating problem, have a professional mobile mechanic come to your location to complete a car is overheating inspection.

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