The thermostat controls the engine’s operating temperature. They open at a certain temperature, allowing coolant to flow to the radiator for cooling. It’s located between the engine's outlet and the radiator. For instance, a valve rated at 195° F will begin to open at 195° F and be completely open at around 215° F.
A stuck closed thermostat results in engine overheating, because doesn’t allow coolant to flow to the radiator for cooling. A stuck open thermostat would not allow the engine to reach normal operating temperature, resulting in (TCC) torque converter clutch and fuel mixture problems. If a customer mentions the vehicle sometimes gets hot and then suddenly returns to normal, suspect a faulty thermostat. A thermostat that sticks intermittently will result in an engine that runs hot and then suddenly cools, returning to normal operating temperature when the valve opens.