Q: Temperature guage stays on cool, heat inside is not very warm

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While driving with heat on, I noticed that it wasn't heating like it should inside the car, not warm enough. Also noticed the temp guage never raised off the cold on a 10 mile trip. Could this be a sensor problem? Or more likely the thermostat needs to be replaced?

My car has 190000 miles.
My car has an automatic transmission.

If your heater is not getting warm enough and the engine is not reaching operating temperature for a long time then your engine most likely has a thermostat that is stuck open. A stuck open thermostat can be verified by a mechanic like one from YourMechanic. The mechanic will start and run the engine to check the cooling system to see if the thermostat is causing the engine to run too cold. If it is stuck open then it will cause the heater to not get warm enough and the engine running too cold will get poor fuel mileage. Have the thermostat tested and replaced then run the engine to see if the engine comes to operating temperature within about 3 minutes running the engine at 1500 to 2000 rpm. Then recheck the heater operation and it should work correctly.

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