456 HVAC problem

I have a buddy who has a 456. He tells me the HVAC blows only on the high setting. The knob doesn't appear to work when he turns it to a lower blower setting. It does blow either cold or hot air depending on whether the stop button for the AC is on or off.
Do you guys think it is the HVAC box or could it be something else I could tell him to check frst. The new HVAC boxes are around $2K...

 

Thanks