With Manual Controls

1. Check for vacuum to the Solenoid Box.
2. Check HVAC Fuse and CLUST Fuse.
3. Move the Temperature Lever to full "COLD" and then full "HOT." Listen for the sound of the Servo Motor moving the valve through its full range.

^ Check for a broken (or partially broken) wire inside of the insulation which could cause system failure but prove "GOOD" in a continuity/voltage check.
^ Check for proper installation of aftermarket electronic equipment which may affect the integrity of other systems.