Heater valves are are open at rest state and closed with PWM voltage from IHKA (AC control unit). When the problem occurs you can disconnect the plug on valves for test purposes.

If the fan inside IHKA fails the temperature inside the unit (where interior heat sensor is also located) warms up after longer drive and can cause lower heat demand than anticipated. You can use smoke or a piece of paper to test if air is drawn into IHKA via the grill on the fascia.