Q: Thermostat stuck

A stuck closed thermostat will cause a reduction of coolant getting into the radiator so heat can be transferred out of the cooling system. The thermostat will need to be replaced if it is indeed stuck closed and the cooling system should be pressure tested to ensure the system has not developed leaks due to the over stress caused by a stuck closed thermostat and overheating.

Note: Never open the check or open the cooling system with the engine hot or the system pressurized as serious injury can occur.

You may want to enlist the help of a mechanic, such as one from YourMechanic, who will have the equipment and training to diagnose the cooling system issue, identify why your car is overheating, repair the problem, and properly fill and bleed the cooling system so it will perform in its optimum range.