I briefly mentioned last week that I was presented with an 'Insufficient Coolant' warning message after starting my Phaeton up, following an overnight cold-soaking down to around -25°C. I was really perplexed by this message, because the car was parked on top of a layer of pure white snow, and there was no evidence that any coolant had leaked out of the car.
About all I can think of as a possible cause is 'shrinkage' at the very cold temperature I encountered. By this I mean perhaps the coolant shrunk as it became very cold, and this emptied out the little reservoir, and the coolant was then trapped in the engine by a non-return valve. This is, however, speculation on my part - engines and stuff like that are not my strong suit.
I opened the cap on the coolant reservoir and looked inside - sure enough, the coolant level was very low. I started the car and allowed it to idle until it reached operating temperature. By that time, the coolant had expanded again, and the warning message had disappeared.
I dropped by my VW dealer once I got home, and they gave me some coolant to put inside the reservoir. The Phaetons (both engines) require the newest 'G12 Plus' (color purple) specification coolant, which is designed to last the life of the engine, although the service manual does allow for topping up with G12 or G11 coolant if G12 Plus is not available.
Below are some photos showing what I found.
The warning message (note the OAT)
What the reservoir looked like when I took the cap off
(Note that the 'MIN' tab is way above the coolant level)
What we added
I'm not really sure if this is G12 Plus or plain G12.
What it looks like when properly filled up