Open the hood. Turn on the AC. See if the fans come on.
And no, the knob working doesn't necessarily mean the blend door is fine. There is foam on the door and it comes apart over time. I've started to have a few pieces come through my vents. So the door will work, but it won't seal all the way. More of an issue with the older cars ('98, '99, etc) simply due to age on the foam.
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The Baby: Darla, 102k, '99 New Beetle GL 5-speed. Bone stock. Acquired 4/6/05. Gave her life for me 8/17/11. RIP 
The Project: Klaus, 180k+, '84 Jetta GL Turbo Diesel. Rebuilt and bringing 50+ mpg. Slightly improved from stock  Acquired 7/12/08
The Baby Brother: Günther, 20th Anniversary Edition GTI 
The money saver: 2001 Jetta TDI GL. 114k. Dead auto, swapped to 3 pedals. Hubby's DD and our trip car
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