If bleeding the radiators does not clear the D6 error, or if the pressure spikes back up immediately after you turn the heating on, you must contact a Gas Safe registered engineer.
Rare, but possible. If the board’s flame-sensing circuit fails, it will report D6 even with a perfect flame. This is usually the last thing to replace after ruling out everything above.
Dust, debris, or a mechanical fault can prevent the fan from reaching the required RPMs. viessmann d6 error
If the switch is not clicking or conducting, it needs to be replaced.
If the switch and fan test perfectly, the engineer will check the voltage outputs from the PCB to determine if the control board needs replacement. Preventative Maintenance If bleeding the radiators does not clear the
to see if any secondary codes are hiding in the "Fault History" menu. When to Call the Pros
Viessmann systems use a digital address system (often via DIP switches) so the boiler knows which device is talking to it. This is usually the last thing to replace
The information in this guide was synthesized from multiple sources, including:
Check the gas supply
Even a small amount of trapped air can act as a blockage, preventing water from flowing freely. This is a very common cause of an intermittent D6 error that appears and then disappears.
In rarer cases, the air pressure switch and fan function perfectly, but the printed circuit board (PCB) fails to read the signals correctly. A damaged microprocessor on the PCB will display a false D6 error. Step-by-Step Troubleshooting Guide