The thermostat in a water-cooled engine controls the temperature of the coolant. The thermostat stays closed when the engine is cool to restrict the flow of coolant. This allows the engine to warm up quicker. Once the engine reaches operating temperature, the thermostat opens up, allowing the coolant to flow freely through the engine passages. The 2003 Ford Focus was in its fourth year since its 2000 introduction and was available with a slightly larger 2.3 liter engine as an option over the 2.0 liter engine. A malfunctioning thermostat in your Focus can cause the engine to overheat.
Open the hood and allow the engine in your Focus to cool so that you will not be scalded by hot coolant. Place a drain pan under the radiator drain valve. Loosen the valve without removing it with an adjustable wrench in a counterclockwise direction. Close the valve once the coolant has finished draining.
Locate the coil pack wiring harness on the driver's side of the engine to the right of the valve cover. Press in on the release tab on the underside of the plug and pull it apart. Remove the brackets from the EGR hose in the same location with a 13-mm and a 10-mm wrench. Push the plug and the hose aside to gain access the thermostat housing.
Pinch together the tabs on the upper hose clamp on the thermostat housing with a pair of pliers. Slide the clamp off of the hose. Pull the hose from the housing with a twisting motion. Rotate the housing to reveal the remaining two hose attachment points and remove these hoses in the same fashion. Remove the thermostat housing from the engine bay.
Remove the thermostat retainer from the housing by twisting out the retaining screws with a torx screwdriver. Remove the thermostat from the housing and insert a new one into the housing using the same orientation. Replace the thermostat retainer and tighten the torx screws.
Position the thermostat back into the engine bay and connect the three hoses removed earlier by sliding the hose clamps back into their original position. Replace the EGR valve brackets and tighten the retaining bolts. Plug the coil pack wiring harness back together until it is fully seated.
Remove the coolant filler cap on top of the engine and place a funnel in the opening. Pour the coolant from the drain pan into the funnel. Remove the funnel and replace the cap. Start the engine and allow it to reach operating temperature. Check for leaks.
