What Tools that can be used to test the operation of the thermostat without removing it from the engine?

To test a thermostat without removing it from the engine, you can use an infrared thermometer to measure the temperature of the engine housing near the thermostat. A coolant temperature gauge or OBD-II scanner can also provide real-time temperature readings to check if the thermostat is opening and closing at the correct temperatures. Additionally, a pressure tester can help assess the overall cooling system's integrity, which indirectly reflects the thermostat's operation.