Vaillant aroTHERM fault F.755

What it means

Incorrect position of the 4-port diverter valve. In heating mode, the flow temperature is lower than the return temperature in the building circuit, or the temperature sensor in the electronic expansion valve environment circuit displays an incorrect temperature.

Likely causes

1. Incorrect position of the 4-port diverter valve
2. The temperature sensor in the electronic expansion valve environment circuit displays an incorrect temperature
3. Flow temperature lower than return temperature in heating mode

Where to start

1. Use the sensor/actuator test to verify the 4-port diverter valve produces an audible switching sound when actuated
2. Check that the coil is positioned correctly on the 4-port valve
3. Check the wiring harness and plug connections