[header] How to tell if your car's thermostat is stuck closed [title] Check your temperature gauge in the car. [step] A stuck thermostat might be to blame if your gauge marker nears the red danger zone within five to 15 minutes. [title] Turn off your car and allow it to cool before opening the hood. 
[step] Unhook your coolant reservoir if you can undo it and pour the antifreeze into a container. Locate your radiator and remove the cap.