R129 Automatic Climate Control Question

[jgl1]

1997 SL 600 (U.S. spec, 129.076)... climate control no longer responding to some commands... request for A/C results in compressor clutch engaging, blower speed is adjustable, as is temperature... but air no longer comes from the center or side dash vents as the vent flaps do not open.

The push button control unit (PBCU) is properly version coded and no fault codes are stored. With use of a M-B HHT in activation mode, vent flap functionality returns.

A fault code check of the instrument cluster reveals one stored fault: B1755, K2 communication, IC to PBCU (N22).

Continuity check of the K1 and K2 CAN lines between the PBCU (N22) and the instrument cluster reveals the lines are intact.

Is it safe to assume that the PBCU is faulty and warrants replacement since the heater box air flap elements are functional when the HHT is connected in activation mode?

Thanks.