Does anyone know how to diagnose whether you have a bad HVAC blend door actuator or if the blend door unit itself is bad? I have a 2005 Ford Explorer with the 4.0l engine. 2 weeks ago when it warmed up here, temps in the 60's, I had switched the temp control knob for the heat over to the cold side for a few days. Once I went to turn the temp control knob back to the heat side when it got cold again outside, I started to hear a "clicking or thumping sound" coming from behind the center-dash console area where the HVAC knobs are located. The clicking or thumping noise I hear happens when knob is moved to the heat side of the control, with no heat coming from the vents, still cold air coming out. I researched this problem on this site, found some info on this blend door-actuator problems other people have discussed, but found no definate answers on how to know which problem I have, bad actuator or bad blend door. Ford Dealership says the actuator is usually the part that goes on these vehicles and it's the cheaper of the two parts. The dealership wants $92.00 for the actuator or $400.00 for the blend door unit, parts only, labor is approx: $580.00 to $620.00 depending which fix is needed. I found a temporary fix on here called "the 10 second blend door repair" where I was able to reach in behind the dash control knob area through the glove box and flip down the blend door hinge on to the heat side and at least get temporary heat in my vehicle. After the vehicle sits parked over night and I go to start it up the next day, the blend door is usually flipped back to the cold side and the actuator starts clicking again and I have to do the 10 second fix nearly every day. With these symptoms I have mentioned here, does anyone know which problem I have, a bad actuator or bad blend door? Any and all help is appreciated, thanks!