From the Tech Support Archives: 2007 Passat with a fogging problem

Here's a car that has trouble with the windshield fogging in cold temps. When the driver gets in the car the windshield is clear, but as soon as the fan is turned on the windshield fogs badly and then takes forever to defog...even with the fan on high. Diagnosing this one means we need to think hard about just how the defroster works in this car.

In this model VW, the AC is disabled below 39 degrees, so AC probably isn't playing a role (or helping clear the fog!). Lot's of condensation inside the windshield can only occur when there's lots of condensation in the air, and in cold weather we know that the air is usually pretty dry. But the driver (and any passengers) would certainly bring moisture into the passenger compartment, right? So how's the fan blowing moisture up onto the windshield? Must be recirculating the moist air that the driver and passenger are creating. The only way that would happen would be if the recirc flap was closed in "recirculating" position and no fresh air is coming in.

Based on this, we'll want to check the recirc flap for proper operation. If it's stuck closed, then we might have solved this one, and all that's left is the actual fix. I'm glad that I'm not the one crawling up under the dash for this job!