With the engine at operating temperature and the ACC set for max temp in economy mode: the coolant supply line to the aux. pump is hot; the lines running from the duovalves to the heat exchanger are warm (likely due to the elevated engine compartment temperature).

Checking the heat exchanger's temperature, via the pusbutton control unit, reveals the temperature to be essentially the same as the vehicle's interior temperature ... less than 90 deg F. Obviously, no engine coolant is entering the heater core.

Fault code check using the M-B HHT and the pushbutton control unit diagnostic mode reveals only the heater supply unit circulation pump (aux. coolant pump) was defective. No other codes are found.

Removal of the existing pump revealed the pump was defective. Installation of a new, functional pump, however, made no difference. So, in addition to the pump, either the duovalve assembly is defective or the pushbutton control unit is not functioning properly when heat is requested. A overhaul kit is not available for the duovalves.