2001 Isuzu Trooper:
Intermittently the ducts will not blow any air at all when it gets really cold outside upon start up. The fan is working it's just not pushing air through the vents in any mode, so apparently the mode door is not opening inside the heater box. I put a box heater on the floor board and everything works fine after it heats up the components. When it doesn't work there is free flow movement from the fan box through the a/c condenser and then it is restricted somewhere in the heater box.