How to Tell Thermostat is Stuck Open by Checking Both Radiator Hose & Using an Infrared Thermometer

How to check thermostat is stuck open by checking the upper radiator hose and lower radiator hose. At start-up, one of the radiator hose should be warm and the other hose should be cool. If both radiator hose feels warm at the first two minutes of start-up, it means the thermostat is stuck open. The thermostat should be in the closed position to allow coolant to warm up faster in the cooling system. This will allow the engine to reach operating temperature quicker and this will reduce engine wear, reduce emissions, and help with gas mileage which can be wasted at start up. You can use a scan tool to see if engine reaches operating temperature. If engine is running cool, the thermostat is stuck open. An infrared thermometer can be used to verify radiator hose temperature.