The Phaeton tech at my VW dealership and I determined through experimentation that the transmission only generates that particular fault code (the one that relates to the torque converter) if the car has been parked outside at night (cold-soaked) at temperatures below -10°C. Furthermore, whatever condition it is that causes the light to come on is momentary - the light comes on when the car is started, but if the fault code is then cleared right away, the fault code will not return.
Based on those findings, we kind of looked at each other and decided to take no action at all for the moment until we can gather further information. This sort of reminds me of the discovery process we followed two years ago when we were hunting down the cause of all the low voltage problems on Phaetons, and eventually discovered it was the B version battery controller. In other words, neither one of us suspects that there is a serious problem with the torque converter, and for that reason, the last thing we want to do is replace it.
I'm in Europe now and hope to get to both Dresden and Friedrichshafen (ZF Transmissions home town) within the next month. I want to gather more information before making any further posts on the matter. For the time being, my advice to any other owners who encounter that torque converter fault code is to just have it noted and documented on a work order (in case further action is required later on), clear the fault code, and keep driving. BTW, the thread that discusses this particular fault code in more detail is here: Does my W12 need a new Torque Converter? There is a different thread for discussion of 6 speed transmissions here: 6 Speed Automatic Transmission Concerns (V8 Phaetons). I am going to beg everyone's cooperation in keeping transmission discussions on those threads, not here on your new topic about triggers for the CEL.
As for the CEL itself, first and foremost we have to keep in mind that the CEL is an emissions control warning light, not an engine health warning light. There is a different warning light called EPC (Electronic Power Control) that tells you if you have an engine control problem, and if you have other serious but non-emission related engine problems such as overheating, low oil pressure, low oil quantity, etc., you will get a plain-language warning in the display between the speedometer and tachometer. See this post for a list of possible plain-language warnings, and also for a list of illustrations of all the warning lights: Instrument Cluster - what do the symbols mean? (list with a photo of every possible symbol that can be displayed).
Keeping in mind that the CEL is an emissions control warning light (mandated by the US Federal Government) and not an engine health light, here is my personal guess at what the primary causes are for CEL illumination. These observations are based on my experience moderating the forum.
1) Loose gas cap - this could allow fuel vapour to escape. The fuel tank goes through its own pressure-test routine on each driving cycle, if a pressure leak is detected, the CEL comes on.
2) Temperature changes in winter - if you park in a heated garage, drive outside to a very cold day, open the gas cap, partially fill the tank, then put the gas cap back on again, you can sometimes get a spurious CEL because the warm air in the tank is shrinking at the same time the pressure test is being carried out.
3) Ethanol - Look in the owner manual - I think the Phaeton engines were designed to run on a maximum of 5% ethanol. Legislators all over the place are mandating higher ethanol levels all the time, either to polish their environmental image and look green, or to keep the campaign contributions from Archer Daniels Midland coming in.
4) Poor quality gasoline - this can be caused by any number of reasons, ranging from long storage time (stale fuel), poor QC at the refinery or during transportation, or flaky regional legislation that governs fuel formulation (e.g. Chicago area in the winter, and W12s stalling). Poor quality fuel causes misfires, misfires illuminate the CEL.
5) Vacuum related things - many of the emission control systems on the car use vacuum. If there is a leak in any of the vacuum systems (a loose connection, damaged hose, etc.) then the light will illuminate.
6) Transmission - I'm referring to the torque converter slip issue for the W12 cars here - but, the only reason that the CEL illuminates is because if the torque converter is found to be slipping (not locking up), this will increase fuel consumption, and increased fuel consumption creates more emissions, therefore the CEL must, by law, illuminate. The fact that the fault condition (the slip) only exists for a few seconds on start-up, before any attempt is made to move the car (take it out of Park) is beside the point as far as the legislators are concerned.
Some of the factors that cause the CEL to illuminate indicate problems in the motive system of the vehicle, and these problems should be further investigated to determine the cause (the fault code). You cannot make a decision about the significance of the CEL warning until you see what the fault code is. Most of the times the CEL on Phaetons that have been well maintained (serviced in accordance with the service schedule) illuminates, it is a nuisance warning. Some specification is not being met, but it does not signify an engine health concern (e.g. gas caps, bad fuel, etc.)
If you encounter a serious engine problem, you will see the EPC light or get a plain language warning in the instrument cluster. The CEL is an emissions control light!