The C4 AAN rad (Item 1 in the diagram) PN is 4A0121251A (251C = Auto trans). The coolant overflow/reservoir tank is Item 37 (PN 4A0121403). Conventional wisdom says don't buy an aftermarket coolant reservoir, buy an OE reservoir from a dealer. Save yourself grief.



Changing from G11 (blue) coolant to G12 (pink) coolant can be done, but you need to totally flush the system, as noted in this discussion:

G11/G12 discussion on AW

The reservoirs typically fail because either the small nipple for the small inlet hose breaks (common) or their is an issue with the outlet nipple (rare) or the float "Switch" (rare). They have changed the design on the small inlet nipple so it is fully (and not partially) reinforced with an internal metal sleeve. I recently replaced my reservoir during a timing belt replacement (coolant was drained anyway) (I had tried to remove the coolant hose but the unreinforced portion of the small nipple just crumbled as I made the attempt = new reservoir). I kept the old one to cut open and finally did that this AM. I thought you might be interested as to what is inside the tank.

Here is the tank cut open to reveal two parts, separated by baffles (with orifices to allow flow between halves, presumably to steady the level around the level float switch):



Here is a closer view of the level switch (on the left) and the small (broken in this case) nipple (on the right). The float ring has a circular magnet that goes over the pointed rod that has something metal (like a wire and resistor "loop") embedded into it and is connected to the two-pin "switch". I presume the magnet changes a resistance or induces a current so that the signal sent to the System Check can be interpreted as either good (OK) or bad (Coolant Level Alarm diagram).



I don't think either the nipple or the switch can be easily successfully fixed. Easier just to purchase a new reservoir from the dealer (in this case OE parts are definitely superior to aftermarket parts).

Now, did you ever wonder what that white triangular(ish) plastic "cap" on top of the coolant expansion tank (see photo)does?


Base photo courtesy of Arizona Autohaus

I *did* wonder but didn't pursue that answer the other day when I posted the cut-away of the tank. I should have investigated further. My bad.

Manny S. emailed me and asked to look at that area because he was losing coolant on track days, even with new tanks and new caps and thought that there might be a connection with the white plastic "cap". This is what I found out:

First of all, the white plastic "cap" does come off (it clips onto the natural polyethylene tank (I don't think it is glued on, at least it wasn't on the "donor" tank). Apparently (as in I am making an educated guess), the coolant tank cap vents when the pressure gets too high, i.e. coolant temps are too high and/or the coolant levels are maybe too high, say on a track day. Audi apparently designed the coolant tank to direct the vented pressure (hot vapours and some hot coolant) in a safe manner, i.e. down and away from anybody near the cap (this would include somebody releasing the pressure by twisting the cap off). This vented vapour and fluid flows under the white plastic "cap" and down the big tube where it enters two slots in the white tube and then out the bottom of the tank. The small white tube that fits in the larger tank tube shows in this diagram (with the white plastic cap pulled off):



Manny's excellent solution was to stick a piece of heater hose onto the nipple end of the white plastic cap (up into the larger tank tube shown with the red arrow in the photo below). He then ran the hose to a catch can and stopped the uncontrolled dripping of coolant. Good idea on a track day.



This might explain why we sometimes can't find where we have a coolant "leak". It might be out of the white plastic cap and through the tank tube to the area underneath the reservoir. In Manny's case, the coolant was leaving wet residue marks on the leading edge of the drivers side strut tower.