A or sensor that isn't reporting flow correctly.
during installation or maintenance. Steps to Fix a D6 Error
: For low-temperature heating setups (like sensitive underfloor heating manifolds), a manual safety thermostat is often strapped to the supply pipe. If the water gets too hot, it trips and passes the fault to the EA1 module.
: Check the external flue terminal for any visible blockages or damage. viessmann d6 error
Debris in the system filter can restrict water movement.
Now, let's get to the story!
If the system pressure drops too low, there is not enough water volume to absorb the burner's heat. A or sensor that isn't reporting flow correctly
Sometimes the error is a momentary glitch.
| Possible Cause | Likelihood | DIY Fix? | |---------------|------------|-----------| | No gas supply (closed valve, empty tank, or pressure drop) | High | Yes | | Air in the gas line | Medium | No (requires purge) | | Igniter (spark electrode) dirty or faulty | Medium | No | | Flame sensor (ionization rod) dirty or faulty | Medium | No | | Condensate pipe blocked (in condensing boilers) | Medium | Yes (thaw/unblock) | | Faulty gas valve or ignition unit | Low | No |
To avoid seeing the D6 error in the future, ensure your annual boiler service includes a check of the external weather compensation settings. Ask your engineer to verify that the sensor housing is watertight and that the entry points for the wiring are properly sealed with silicone to prevent moisture from traveling down the cable into the electronics. If you'd like to narrow down the cause, tell me: If the water gets too hot, it trips
If water squirts out of the vessel's air valve during testing, the internal diaphragm has split, and the entire expansion vessel must be replaced.
Sudden electrical spikes can damage the sensitive burner control unit PCB. A surge protector safeguards the electronic board.
This and subsequent steps must be conducted by a certified Gas Safe engineer. Open the boiler casing and locate the air pressure switch. Inspect the thin silicone Venturi tubes for signs of moisture, cracks, or dirt. Disconnect the tubes, blow through them gently to clear any condensation, and reconnect them securely. 4. Test Voltage to the Fan