A couple of weeks ago my wife came home in her '01 TT with clouds of anti-freeze billowing out. It started about two blocks from home. I put the car in the garage and let it cool down. When I looked under the hood, I found that the temperature sensor was popped out of the hose fixture! The plastic C clip was still in place, apparently a build up of pressure pushed the sensor out.

I looked around and found no other problems. Since the car is 10 years old, I figured that the thermostat was at fault. I ordered a new temp sensor and thermostat and started to figure out just where the thermostat was located. I finally found it conveniently located under the intake manifold where only a spider monkey or an 8 year old child could get at it.

I was out of monkeys and 8 year olds, so I started to photograph, remove, label, photograph, remove, label, etc until I got to the thermostat housing. I removed the housing, cleaned up the block and the housing and replaced the thermostat. I put the rest of the front end of the car back together. I had no 'extra' parts! Good sign.

I then went after the temp sensor. I removed the battery cover, the intake cover, the intake hose, the lower hose on the intake box, the fascia on top of the engine, etc. I pulled off the plastic C clip, reached in the sensor space and pulled out the O ring. I then placed the old O ring on the new sensor, pushed the sensor back in its space and slid the old C clip back on, and snapped the wiring connector back on the top of the sensor.

I added water to the reservoir, started the car and let it warm up. I added more water as needed. The car warmed up but the radiator fan never came on. I shut it off.

I then traced everything that I could find in the wiring diagrams for the temp sensor and the radiator fan. I looked at every fuse in the fuse box. Then I found the three big-***-fuses in the box on top of the battery. Here was the actual problem! The fuse that controls the radiator fan had built up resistance over time and was not passing current. I pulled each one out and cleaned it off, and slid it in and out a few times to re-establish the connections.

I started the car again, this time the fan came on and the car ran as expected. It seems that the temp sensor nor the thermostat were at fault. It was the fuse. I don't think it was a bad thing to replace the thermostat and it was a big box of fun replacing it.

The temperature sensor is another tale altogether. I only replaced the sensor and I replaced it with a non-Audi part from some Auto Zone or something like that. Bad idea. The sensor worked loose again and caused the car to lose fluid again. Fortunately, it happened at a convenient location and I was able to get the car home without a problem.

I purchased a 'real' Audi temperature sensor, O ring and plastic C clip. On inspection, I noticed that the brass part of the sensor for the Audi version had a nice flat area for the C clip to sit, while the aftermarket sensor's brass upper edge was rounded. I think this rounding contributed to the second failure.

The outcome of all of this is: keep an eye on those fuses on top of the battery and if you have to replace the temperature sensor, get a real one and also replace the O ring and C clip.

I still have to deal with draining and replacing the coolant.

 

Keep a close eye on the waterpump too. On my 2001 when I pulled it to change the timing belt, the impellor was completely gone and in pieces! The OEM one is plastic and the new one is metal. Cars runs cool now.