I've been having issues with the temp gauge reading all over the place and decided that the ECT was to blame. I took care of that and then went for a drive to test it out.

On the initial drive, things went as expected. The engine warmed up and the needle climbed to the halfway mark and stayed there for a while. I had heat in the cabin, which was never a problem before. Here's the weird part. Then the gauge proceeded to go all the way back down to one mark above cold. Heat in the cabin is still good.

I originally ruled out the thermostat since I always had heat and the temp didn't seem to fluctuate with moving or stopped at a light. What's my next move?

 

when you turn the heat on it should drop the gauge. Im not sure if it should go back that much tho?

 

Heat on or off didn't seem to affect the gauge reading . This is really frustrating.

 

Heater on/off has never affected my gauge.

 

try replacing coolant temp sensor mine did this also ,,sensor fixed it

 

Replacing the coolant temp sensor corrected this issue for me. There are a couple different type sensors (indicated by their cap colors). Make sure you got the right type.

 

I replaced the sensor as previously mentioned because I thought that it would solve the problem. All of the symptoms seemed to indicate that everything was working fine aside from the sensor.

There is the possibility that I received a "new" bad sensor. I assume that the sensor is just a thermistor should be able to test it by monitoring resistance at various temps. I suppose that there is some diagnostic I could do if I had VAG-COM.

Any other thoughts?