my car is taking forever to warm up. When it finally gets to the right temp and i get on the highway, the temp fluctuates(sp?)between dead center and the cold side. I also have the opposite issue, in warm temps, the temp fluctuates between dead center and hot. STFA found that the temp sensor goes bad and causes the temp to go down, but I haven't read anthing about the temp going up.
Could it be a bad thermostat?
TIA

Thanks

and had alot of pressure.

change it on a morning, before the car has been driven around.

The only bummer is that I had to remove the intake piping to get at it. The Audi dealer in Flemington charged me $7 + change with the o-ring seal. I cleaned out the throttle body while I was in there and wow, what a difference in the idle. Smooth. Good luck.

Sorry to steal the thread.. but I figure this is kinda related. The car used to stay pegged dead center on the temp gauge but recently in traffic or speeds over 75mph it rises to the first fat bar to the right of center. Turning the heat on brings it back to center. I tested the wires going to the aux fan and neither high temp or turning on the A/C sent any voltage to the fan. I think the engine temp sensor is working fine because I have no codes and in the Measuring blocks(vagcom) if you start the car the watch the temp it rises normally. Will a bad thermo switch on the rad. keep the fan from turning on with the A/C. My guess is no. I'll get the car tomorrow to dig into things further.
thanks

Press Recirculate and the UpButton(directs the fans to the windshield). 1c should pop up in the left screen of the climate control. Then press the temp up button until you reach 51c. Then press recirculate again. Now the left screen should show the coolant temp in degrees C.

My A4 is overheating, and once the coolant gets to 90c in Block 51 it stops increasing even though the temp gauge continues to rise. I'll be getting a new coolant temp gauge Monday. Hope this helps.

the temp stayed @ 90c the whole time.
Do you have a list of other read outs that this can be used for?
TIA

that didn't fix my problem. Oh yeah this is superslow90q I'm just using a different computer. I'm going to try bleeding my system again today. I'll let you know if figure anything out. I hoping its not the t-stat because you have to remove the timing belt to replace. And its less than a year old(put a t-belt and waterpump on it)

Bap-geon was 19.95. I'm talking about the blue top sensor. 5bucks sounds too cheap!

i did the t-belt, water pump and t-stat a little over a yr ago.
I bled my system, had the radiator tested for clogs, and still not solved. I'm gonna try the sensor anyway,(it's cheap and easy enough), if that doesn't help, then it's time to change the t-stat.

Sounds very much like a bad sensor or sensor lead from the secondary connection backwards to the ECU. The secondary or additional connector wire can go bad under the dash with the wire wear problem some have experienced. The ECU can't control the gauge if it's getting bad info from the sensor.