The first thing I would do would be look at the switch valve block circuitry on a wiring diagram.

In general most heater/AC systems default to defrost for safety when out of control.

The controller looks at the circuit by watching current flow through dropping resistors inside the control side of the circuit (usually the ground side). Thus a code for a actuator coil element such as the valve block is likely to be noticing no current. It will also posibly shut out at too much current (depending how smart the controller is). I would look at both sides of the circuit especially the fused power side!

It is my guess that the problem will be the circuit not the component with it being intermittant. I would check the fuse, the grounds, and current ramp the circuit while changing duct position to view all the coils for differences.

Unfortunately the most likely weakness in the circuit is probably the controller itself.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician