How to Replace a Thermostat on a 2003 VW Jettaby Russell Wood
The thermostat on a 2003 Volkswagen Jetta opens and closes depending on the temperature of the coolant. When the thermostat fails, it gets stuck in either an open or a closed position. Either way, the coolant will store too much heat, and the engine could overheat as a result. If your Jetta seems to be overheating, one affordable option is to replace the thermostat. The thermostat is inexpensive, and replacing it is a straightforward procedure that you can do yourself using basic tools and materials.
Park the Jetta on level ground and allow the engine to cool down completely. Lift the hood and prop it open. Take off the radiator cap with your hand. Put the large drain pan under the radiator and open up the drain valve on the base of the radiator with your hand. Tighten the valve once all of the coolant has drained.
Locate the thermostat housing on the front of the engine block, connected to one of the radiator hoses. Unbolt the housing with the 3/8-inch ratchet and socket. Pull the thermostat and its O-ring out of the housing.
Install the replacement O-ring in the thermostat housing by hand. Insert the replacement thermostat into the housing, then bolt the housing to the engine with the 3/8-inch ratchet and socket.
Fill the radiator with the 50/50 pre-diluted coolant and reattach the radiator cap. Start the engine. Turn the heater to its highest setting and run the Jetta until the temperature gauge reaches the halfway mark of the gauge. Let the engine cool for at least two hours.
Remove the radiator cap. Add any necessary coolant to the system. Reattach the radiator cap and close the hood.