The thermostat in your Audi A4 is a small, but crucial, device for the proper operation of your vehicle's cooling system. Located between the radiator and the engine, it moderates the flow of coolant into the engine by releasing it when the engine need it the most. The thermostat is designed to open its valve once the temperature of the engine reaches about 93.3 degrees C. Its purpose is to keep the engine running at a consistent temperature, allowing your Audi A4 to operate at peak performance.
Raise the front end of your Audi A4 by driving up on a set of car ramps and placing your vehicle in park.
Place a 4-by-4 inch wood block behind one of the rear tires to keep the vehicle from rolling backward.
Use a socket wrench to remove the pan that covers the bottom of the engine.
Locate the red plastic cap that was behind the pan that you just removed. This red cap is your radiator drain plug. Remove the radiator cap from the top of the radiator and then place a drain pan under the red drain plug and turn it with a pair of pliers to allow the coolant to drain completely from the radiator. Once the coolant has completely drained, turn the drain plug to close it off. Be careful when using the pliers on the drain plug that you do not snap the plug off.
Slide under the vehicle and locate the thermostat housing that is sitting just above the oil pan drain plug. Pour the drained coolant from the drain pan into an empty plastic container and label the container so that you do not reuse the expelled coolant. Place the empty drain pan under the thermostat housing before you attempt to remove the housing to allow the coolant from the engine to drain into it.
Remove the 10mm bolts that hold the thermostat housing on the engine block with a metric socket wrench. Do not lay directly underneath the thermostat housing as the coolant from the engine will begin to drain as soon as you loosen the bolts on the housing. Keep a towel or cloth with you to clean off any spilt coolant .
Remove the thermostat housing from the engine and lift the old thermostat out of the housing. Use a piece of emery cloth to clean the face of the thermostat housing and the engine block where the housing is mounted from any remaining gasket material or debris.
Spread a layer of gasket seal around the face of the thermostat housing and place the new thermostat on the housing, with the thermostat spring facing out from the housing. Position the housing back on the engine block and secure it to the block with the bolts.
Replace the pan the sits across the bottom of the engine of your Audi A4 and then refill the radiator with coolant and put the radiator cap back on the radiator. Remove the wood block from the back tire of your vehicle. Turn the engine on and drive the Audi A4 off of the car ramps. Allow the engine to run at idle for a few minutes to verify that the thermostat is working properly and then shut the engine off.
Allow the engine to cool off before ever attempting to do any mechanical work.