Well I just went through this. If you have VCDS drive around with someone monitoring the group 00. The coolant "temp" is on that line. It goes from a high to low value as coolant gets hot. At operating temperature it should be about "74" (74 what I have no idea). Mine was fine most of the time, but then jumped erratically on occasion (3 times in 20 minute drive). Also monitor group 01, which gives coolant temp in C - should be just shy of 90C
When you get home, check coolant temp with a meat thermometer (open cap slowly). Should be 87C or so. If coolant is too cool, then prob code is from t-stat not working properly. But much more likely sensor is bad.
The one I just replaced in our 2002 was a black one - apparently notorious for failing. Also for replacement buy from VW, apparently aftermarket ones also let out the ghost quite quickly.
Cheers.
Jeff