[CHPO-18,CHPO-24,CHPO-30,CHPO-36,CHPO-42,CHPO-48] I have a CHPO-24 -- the 'tabletop' outside unit and the ladder diagram. Ongoing problem is the unit goes into lockout whenever it tries to enter defrost mode. Second problem: it rarely tries to enter defrost mode. Defrost is triggered by an air-pressure differential switch that is supposed to detect ice buildup on the evaporator coils. In reality, the coils must be a solid block of ice for the swich to activate. If and when the pres-diff swich closes the reversing valve starts to change, the fan relay clatters a couple times, and lockout occurs. However, if I put a jumper across the press-diff switch terminals, normal defrost cycle proceedes. I have replaced the controller board to the tune of $220 with no change in behavior. Does anyone know if there is a timed defrost cycle built into this model? Any new information greatly appreciated.