What would cause coolant temperature gauge to stop working?

"i can only guess - but the coolant temperature sensor failed very often on those models

since the audi is CAN-bus controlled, i would use the A/C or my CAN-bus reader to get the actual error codes. i didn't found a good source for the A/C trick in english - only german sources - here is a google translated page: CLICK"

It's a shot in the dark, but I would start by checking the thermostat first.

to verify if the sensor is what's causing the problem, disconnect the connecter from the coolant temp sensor, and jump a paper clip across the terminals. turn the vehicle on after doing this, and see if the gauge has moved in the dash at all. try the same thing with the connecter disconnected. if the gauge moves at all, then it is the sensor itself that has failed.

On the Audi the sensor is your problem

"ECT (engine coolant temp sensor)

the ECT responds to change in coolant temp. by measuring the the coolant temp, the ecu knows the average temperature of the engine .

the ECT sensor is needed to controll many functions such as fuel injection, ingnition timing,variable valve timing, transmisions shifting ect."

"I had the same problem happen today on my 92 Gmc sierra. It turns out the harness clip on the Coolant Temperature Switch snap off due to age I suppose. Anyway, I connected it and secured it until I can get a harness connect replacement (which for some reason is rather high in price for what it is). Cranked it up and took it down the road and the gauge works, it isn't loosing power when I come to a stop (it had stalled completely a couple of times earlier this morning), and I still have a little eratic shifting cause my tranny needs to be replaced. Ironic the same symptoms and problem here...wow. My radiator hose actually constricted a couple of times too. I constricted from the thermostat neck all the way to the radiator too.

Lee"