I have a 2003 Passat GLX, V6 that always blows hot air through the ventilation system. The A/C works and will cool, but once the thermostat is satisfied, hot air blows. EVEN with the Climatron off, hot air comes from the vents. ANY suggestions?
Inside the engine compartment, attached to the firewall, are two hoses. They should be right next to each other and, ultimately, they will connect to the head and/or the coolant system. These hoses, on the pass compartment side of the firewall, connect to the heater core. They provide hot water into the heater core...then your blower fan blows air across the core to heat it.
Follow those hoses inside the engine compartment and see if there is a valve in line with one or the other of them. That valve is what stops the hot water entering the heater core when you push the lever or knob off of "hot" to "cold". It's likely vacuum operated and either isn't getting vacuum or it's stuck in the "hot" position.
Sadly, with newer cars, this valve is often located under the dash, where you can't easily get at it.
However, it's possible you have a vacuum leak that's causing this problem. When you accelerate on the freeway (like up a hill) does your air stop blowing out of the dash and start blowing out the defrost? That's a sure sign of a vacuum leak.
At any rate, if the valve is under the dash you'll need a book to figure out where it is and how to troubleshoot it.
Thanks Thomas and FKH161 for the helpful information. It does not act like a vacuum leak (I have a truck with that problem, exactly like FKH161 described). I recently ordered the Bentley's Manual for the Passat, and will f/u on the valve suggestion.