What I was getting at, was the gauge isn't necessarily lock step the same as the ECU's input. The ECU time stamps the coolant temp at start up, then monitors the rate at which it climbs. Honestly, though, the TDIs, even the CR engines, seem to be pretty lenient about that sort of thing. I have seen a few set a P0128 (coolant temp below operating range) but it may be one of those things that if the temp has passed the threshold after the initial check, then the ECU is satisfied for that drive cycle, and if it cools back down (which, going down a mountain it certainly would), the ECU does not care.
But since you asked, I looked it up:
For the CJAA engine, the test criteria is start up coolant temp is below 60 C, but above -10 C. If it is TOO cold, the ECU is told to ignore this, as the logic knows the engine may NEVER get up to temp as it ideally should. Then the time is 900 seconds. It shows it wants to see an increase of 3K at the CTS, I would take that to mean 3k Ohms value change.
So looking at the CTS resistance chart, let's say you started the car at -5 C, it should be around 7K Ohm, and at 15 C it is around 3K. This difference of 4K Ohm so long as it occurred in 900 seconds, then the ECU's coolant temp logic is satisfied for that drive cycle. The drive cycle will not "reset" until the coolant is back down below 60 C and the engine is turned off.
So, in the Queen's terms, if you started your engine up cold with the coolant around 20F, and after 15 minutes it was around 60F, the ECU is happy, and will not flag a P0128 DTC. I can tell you though that most any TDI can get through that test easily. And once that is done, you can cool that engine back down to a block of ice if you want, so long as the engine has not been shut off, the ECU thinks everything is normal.
So, you could certainly still have a weak or (by most accounts) a "bad" thermostat and still not have the P0128 DTC setting, as the test criteria is not hard to satisfy at all.
