1999 Audi A6 Thermostat

I assume you Audi has the V6 engine in which case the thermostat is located on the left side of your engine when looking at it from the front just below and inboard of the cam sprocket. You need to remove the the plastic cover for the sprocket( on clips). To replace you will want to drain some of the anti freeze from the engine ( do not let the antifreeze pour over your timing belt or it will eventually destroy it) remove the serpentine belt, remove the powersteering pulley(3 allen head screws and pry or pull off) the thermostat is located behind the goose neck going to your left with the radiator hose attached (just above alternator) the housing is held on with 2 bolts with 10 mm heads if your good and careful you can slide the housing out from behind the timing belt without disturbing it replace the thermostat and the O ring ( make sure the bleed nipple on the thermostat is at 12 oclock and reassemble remember to bleed the air out of your system when you refill with antifreeze