A sticking thermostat is a serious problem. The thermostat is one of the components of your car's cooling system. The thermostat manages the amount of coolant that is distributed through the engine. If a thermostat is "stuck" that means it is over-cooling or not cooling the engine enough. One sign of this is an erratic temperature gauge where the engine goes up and then down in thermal temperature. An occasional "check engine light" can be another sign of this problem. To avoid a trip to a mechanic, there are a number of steps you can take to fix the problem.
Park your car on a flat surface and have the emergency brake on.
Wait until the morning or a few hours after driving. You never want to work on a hot engine unless the car overheats while driving.
Open your car bonnet after your car has had some time to cool off. Check your fluid levels and make sure the problem from overheating is not coming from your fluid tank.
Find the thermostat. The location of the thermostat depends on your car. Although it is usually near the coolant fluid tank and the car engine, you should consult your owner manual for the exact location.
Remove the radiator cap. Again, this should only be done when the car has cooled off from driving. If you do open it immediately after having your car on, there could be a pressurised burst of coolant.
Have someone else start the car for you. Look carefully, at a distance away from the open radiator cap, to see if fluid is being allocated to the engine. Have the person who started the car also notify you of the engine's temperature.
Turn off the engine and feel the two thermostat hoses that connect the fluid tank with the engine. Check the top and bottom hoses. The lower radiator hose should be hotter than the top. If somehow the top one is hot then there could be blockage issue.
Let the car cool more and disconnect the hoses from the engine. See if there are any physical blocks in the hoses and make sure the valves in the hose, which open and close when fluid is needed in the engine, are able to move up and down. If they cannot move up and down then they are the main problem.
Replace the valves in the thermostat hoses. Follow this step only if the valves are the main cause of the sticking thermostat. You can either replace the valves with new ones or work the valves back into a position where they can move up and down more freely.
Replace the thermostat. This is the last thing you want to do, but if the hoses do not show a blockage or coolant hindrance then the thermostat itself may be malfunctioning.
There is a difference between a thermostat being stuck open and stuck closed. Stuck open means too much coolant is being allocated to the engine, causing the engine to be abnormally cool. If the thermostat is stuck closed, that means not enough fluid is going into the engine and the engine will overheat.