A stuck pump, airlock, or blocked filter can cause the flow switch to trigger a D6 error because water isn't moving through the heat exchanger fast enough.
Loose or corroded cables between the fan and the main control board (PCB) can disrupt signals. viessmann d6 error
On systems using Vitotronic 200 controls, D6 specifically points to a flow switch issue, indicating that the minimum required flow rate has not been reached. A stuck pump, airlock, or blocked filter can
The precise definition of a D6 error varies slightly across the Viessmann product range: The precise definition of a D6 error varies
If the fan and flow switch are working correctly, the control board itself may be misinterpreting data. Troubleshooting and Solutions
Dust, debris, or a mechanical fault can prevent the fan from reaching the required RPMs.
It often signifies that the fan speed is too high or too low at startup, preventing the ignition sequence from beginning safely.