My car is overheating but I changed the thermostat and the water pump. I noticed it runs hot when I'm on the road and the heater doesn't work. Then when the heater kicks in hot, the temperature declines to normal. Would a sensor do that?
No. The coolant isn't circulating through the heater core. The level could be low but more likely the cylinder head gasket is leaking. When combustion gases pool under the thermostat that can prevent it from opening. Feel the upper radiator hose and it will still be just warm when the engine is running hot. Thermostats have to be hit with hot liquid to open. Hot air won't do it. Your mechanic can do a chemical test at the radiator / reservoir cap. He will draw air through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present that liquid will turn bright yellow.