The thermostat for the 2000 Audi S6 4.2L V8 is located on the front part of the engine block, directly to the left of the fuel pressure sensor. It is held in place by two 13mm bolts.
Below are the steps on how to replace it:
1. Allow the engine to cool down.
2. Locate the thermostat housing, which is located on the front of the engine block. It will have two coolant hoses connected to it.
3. Remove the two 13mm bolts that hold the thermostat housing in place.
4. Remove the thermostat housing.
5. Remove the old thermostat from the housing. Be careful not to damage the O-ring.
6. Clean the thermostat housing and install a new O-ring.
7. Install a new thermostat in the housing, paying attention to the orientation of the thermostat.
8. Reinstall the thermostat housing and tighten the 13mm bolts.
9. Refill the coolant system and bleed any air bubbles.
10. Start the engine and allow it to reach operating temperature. Check the engine coolant temperature to make sure the thermostat is working properly.
