Replaced coolant temp sensor last week, it cured my hard-start hot and fluctuating idle problem. Now idles 750 hot 1200 cold and no cel. Replacement is simple if you can get your hand down to it, I'd recomend pulling the plug and looking at the top of sensor before buying it, passenger side below intake hose with four wires coming out of it. for some reason the auto parts stores have the green (square on one end round on the other) one listed for my '99 A6 2.8, but mine takes the blue square one, if you have a vw/audi specialty store in your area get it there, mine was $12 with o-ring, napa wanted $35. you can also get from ecs. There is a clip that you need to pry off to remove it, DON'T LOSE THIS! Putting it back on was the most difficult part of the job for me, as is hard to get hand down there, don't forget to remove old o-ring, hope this helps.