The cooling system's thermostat controls the engine's operating temperature. It opens at a specific 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 begins to open at 195° F and is completely open at around 215° F.
A stuck closed thermostat results in engine overheating because it doesn't allow coolant to flow to the radiator for cooling. A stuck open thermostat does not allow the engine to reach normal operating temperature, resulting in (TCC) torque converter clutch and air-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 results in an engine that runs hot and then suddenly cools, returning to normal operating temperature when the valve opens.