Ok, let me preface this all by saying I have searched and searched and searched all Audi Forums and google, and can't get the info I am looking for.

I have an 06 A4 that I bought about 3 weeks ago. A week ago it through a CEL of p2181 cooling system malfunction at highway speeds. My temp gauge had always sat a little over the 1/4 mark, and I saw it closer to the 1/2 mark and freaked out. Not thinking I cleared the code, and went to the Audi dealership. They charged me $80 and said they couldn't diagnose the problem since there was no code in the computer, and they couldn't reproduce the CEL. 2 days later it threw the code again at highway speeds.

Paying attention to the temp gauge over the past week I have noticed a drastic fluctuation in it's readings. It does not appear to be related to being at speed or idle, the gauge just seems to fluctuate quite often between the 1/4 mark and the 1/2 mark. Heat/A/C seem to be fine. The car also seems to take awhile to warm up(3-5 mins while driving), but this is my first Audi so I don't know.

Long story short after all my research it seems to me the likely candidate is the Coolant Temp Sensor...correct me if I'm wrong. I think if it was the thermostat I would see it be more of a change be directly related to driving or idling. Also I'm assuming it would be overheating, which it doesn't at all.

So, I would like some input on that...and also if someone knows of a writeup for changing the sensor. I can find pictures and writeups for the 1.8t engine, the 2.8l v6, basically every model but the b7 A4. I can't imagine that changing out the CTS would be something worth taking to the dealership. Any help would be much appreciated.

 

I would replace the thermostat assembly Your car is running too cool.

 

Just curious, did you ever find anything out on this. I have an 06 b7 2.0t with 178k that just started doing the same thing.