Our AAN's temperature is controlled by a conventional thermo-spring based thermostat that keeps the coolant in the block until it reaches 87 deg C, then it opens, letting coolant be pumped to the radiator where it is cooled. These thermostats often break (the brass bar holding the metal diaphram and spring breaks away from the housing). This fails open, which, although inconvenient (no interior heat), the engine is protected from overheating. The thermostat rarely fails closed. This is bad since it will cause the engine to overheat and likely warp the head (bad bad bad). As a result, it is important to monitor the engine coolant temps while driving.

SJM Autotechnik shows us the correct temperature range on the water temp gauge:


My Canadian 93 water temp gauge sits just under the 90 deg C mark when up to temp.

One "interesting" (read: Pain-in-the-arsh) fact is the C4 UrS-car water temp gauge is driven by the multifunction temperature switch (MFTS). Sometimes the MFTS fails so you really don't know if the low temp is related to the MFTS or a stuck-open Thermostat. The way to confirm one or the other is to access the Climate Control Diagnostics and check what the Channel 51 reading is with the heater on. If the gauge reads low and Ch.51 says 89 deg C or so, the MFTS has failed. However, if the gauge reads low and Ch. 51 reads low, e.g. 50 deg C, and you have been driving (or idling) for long enough to warm-up the engine, then the thermostat has failed open and you need to replace it.

Here are the PNs for the thermostat and cover, etc. Note that the thermostat PN covers a very wide range of inline 5 Audi engines.

PNs:
Thermostat - b 069 121 113 - 87 deg C (note: check action in hot water with a thermometer to make sure the new T-stat functions properly)
Seal = N 901 368 02 (might come with T-stat)
Cover (housing)(if cracked) = 074 121 121 B


Make sure the thermostat looks like the one on the left, not the V6 one on the right:


If your T-stat housing (cover) is old, you might want to replace it. The new one will look like the one on the left which has clips to make sure that you install the T-stat with the bar vertical, not horizontal:

Photo courtesy of Mr. Mojo.

Now, the procedure:

1. Start car
2. Drive car onto ramps
3. Stop car, block rear wheels
4. Open hood, remove cap from coolant reservoir.
5. Go under car and remove belly pan if so equipped.
6. Locate rad drain plug (Bottom right side, blue X) and spigot
7. Connect hose onto drain plug spigot
8. Place bucket under hose and open blue X plug (turning counter-clockwise). Drain until bucket is full. Close plug (turning clockwise). Remove hose
9. Locate thermostat housing under intake manifold, near front of engine. (It might be difficult to see but it IS there, AND you can get at it without removing the PS pump)
10. Disconnect throttle body hose at throttle body and shove towards back of engine to get some room to work.
11. With whatever combination of stubby rachet, extension, universal joint, 10 mm socket or 10 mm wrench (check - this is from memory), loosen the two bolts holding the T-stat housing to the block.
12. Place bolts in a safe place and gently pry housing from the block. Inspect housing for cracks. If cracked replace with new plastic or find a good used metal one from an older I5 Audi. If good, shove the housing and hose out of the way.
13. Note orientation of T-stat in block. Cross bar should be vertical as per 20vt90q's diagram from pg 66/93 of this AAN Bentley Manual


This allows air to escape from the engine, negatiing the need to purge the system on start-up after you are done.

14. Pry/pull T-stat out of block (don't worry about breaking it, its already broken) SpeedTek (Jeff M) suggests that you might want to remove the O-ring first since the O-ring might jam the thermostat into the block and prevent it from being pryed out (thanks Jeff).
15. Clean block area where T-stat lives with a rag to remove any debris.
16. Place new T-stat in block with cross-bar facing you and vertical. Air hole goes up.
17. Put new O-ring (seal) in T-stat housing and place housing in the correct position.
18. Start bolts by hand.
19. Tighten with combination of sockets, rachets and wrenches, as required. There isn't much room so try not to round the bolt heads off as you tighten.
20. Refill coolant reservoir with contents of drain bucket (or add new coolant - you may want to flush the system too - but that is an extra, not dealt with here). Leave reservoir cap off.
21. Reconnect throttle body hose (and any related wires you may have moved out of the way).
22. Start engine and look for leaks, run engine until thermostat opens (at around 87-89 deg.)
23. Look for leaks (watch out- the water is hot now).
24. Stop engine and replace belly pan (if so equipped). Replace the reservoir cap.
25. Remove tools from engine bay and lower hood.
26. Remove blocks from rear tires.
27. Back car off ramps.
28. Clean up and crack beer.
29. Take a good swig.
30. Say "Ah..." and Belch (optional)
31. Anticipate next problem. Its an old S-car.

If I've forgotten anything, let me know and I will edit.