On the way back from a weekend trip, I noticed the water temperature jump from nothing all the way upto halfway (Where it should have been, I had been in the car for over an hour).

It happened right after I turned on the air conditioner, and the problem didn't come back after that, even when turning on/off the a/c to try to recreate it. I'm not sure if the temperature gage was malfunctioning before turning on the a/c and that fixed it, or the a/c caused it to dip and then come back and I just didn't notice until it hit bottom.

Has this ever happened to anyone else? Is this indicative of a bigger problem or nothing to worry about?

 

Two possible reasons...

1) if the gauge is moving around, lets say from lower quarter to middle, and drops to lower values while driving on the freeway, and raises back up, then it probably is a stuck open thermostat.

2) If the gauge moves around irratically, from zero to middle while not related to driving condition, then 9 out of 10 times it is the coolant temperature sensor.

Replacing thermostat is complicated. Basically entire front end removal including timing belt and water pump.

But the coolant temperature sensor replacement is a breeze. The part cost is about $20 from the web or $40 from the dealer. It will probably take you about 30 minutes to replace the sensor. Let me know if you want to know how to replace it. I can give you a quick DIY.

 

i had the same prob a few months ago. turned out to be the ect sensor as hxgaser pointed out.

 

Interesting, I'll definitely keep an eye on this. Is there a wiki for common audi a6 issues and solutions? Maybe i should start one.