98 Passat - Probs with Coolant Temp/Level Light
 

I have a 1998 Passat GLS with the 1.8l four cylinder turbo engine. Car has 67k miles on it. The problem is that when it gets a little cold (and we're talking CA weather here, so in the over all scheme of things, I'm talking about pretty mild weather), the coolant temperature & level warning light comes on, the audible alarm plays for a while, then the light stays on for the duration of the trip. The problem is intermittent. The coolant level appears fine (when the engine is cold, it's about half way between min and max on the overflow tank), and the car's been checked out by the dealership. They didn't find any apparent problems, and replaced the temp sensor (I think; I'm borrowing the car from my in-laws, who are traveling). The problem went away for a couple of weeks, then came back. Engine starts and runs fine even when the problem is happening, there's no evidence of coolant leakage under the hood (i.e., that white crusty residue) or in the oil, the coolant is a bit dirty, but looks OK.

Any thoughts on what could be causing this? Thanks!