Sometimes the fault is a temporary logic glitch.
This is the most likely culprit. The thermostat connects to the boiler via a 2-wire or 3-wire cable.
| Step | Action | |------|--------| | 1 | Check other gas appliances work | | 2 | Ensure gas meter has credit / valve open | | 3 | Reset the boiler | | 4 | Thaw any frozen condensate pipe | | 5 | Call a heating engineer if none of the above works |
Disclaimer: This guide is for informational purposes only. Always refer to your Viessmann boiler manual and follow local safety regulations. If in doubt, contact a certified professional.
Troubleshooting the Viessmann D6 Fault The D6 error on a Viessmann boiler typically indicates a problem with the air pressure switch or the fan. This fault occurs when the burner control unit detects that the air pressure switch has not closed or opened at the correct time during the ignition sequence. 🔍 Root Causes
Blocked Flue or Air Intake: Obstructions like bird nests, leaves, or snow can prevent proper airflow.
Faulty Fan: The fan may not be reaching the required RPM to trigger the pressure switch. viessmann d6 error
Defective Air Pressure Switch: The switch itself may be stuck or have failed internally.
Damaged Pressure Hoses: The small silicone tubes connecting the fan to the switch may be split, kinked, or clogged with moisture/debris. 🛠️ Diagnostic Steps
Visual Inspection: Check the external flue terminal for any visible blockages or damage.
Check Hoses: Ensure the pressure hoses are clear. You can use a toothpick to gently clear the barb connection on the inducer motor if it is clogged.
Reset the Boiler: Sometimes a temporary glitch causes the code. Try a hard reset via the control panel.
Professional Testing: If the error persists, a gas-safe engineer should measure the voltage to the fan and test the pressure switch's continuity. ⚠️ Safety Warning Sometimes the fault is a temporary logic glitch
Do not attempt to open the boiler casing or modify internal gas/electrical components yourself. Incorrect work on heating systems can lead to life-threatening accidents and will void your warranty.
If you're in the US, you can contact the Viessmann Tech Line at 844-649-5886 for authorized support. If you'd like, I can help you: Find a local authorized installer
Look up the specific manual for your model (e.g., Vitodens 100 or 200) Explain the reset procedure for your specific control panel Just let me know your boiler model! Viessmann Vitodens 200-W and 300-W fault codes
Q: Is the D6 error dangerous?
A: No. The boiler will safely shut down. It does not indicate a gas leak or carbon monoxide risk. But you will lose heating and hot water.
Q: My boiler shows "d.6" not "D6" – is that the same?
A: Yes. Some Viessmann models use lowercase letters. d.6 = D6.
Q: Can I bypass the D6 error to get heat?
A: No. The boiler’s safety logic prevents operation when bus communication fails. Do not attempt to hot-wire the boiler – this is extremely dangerous. | Step | Action | |------|--------| | 1
Q: Does the D6 error affect the hot water only or both heating and hot water?
A: Both. The boiler cannot interpret any demand signals from the user interface, so it will not fire for central heating or domestic hot water.
Q: Will resetting the boiler every day fix it long-term?
A: No. If D6 returns after each reset, a component is failing. Resetting is a temporary workaround, not a fix.
Provide the engineer with:
Using a multimeter set to DC volts:
A registered engineer will check:
Low pressure points to a gas meter regulator fault or blocked gas filter.
Low gas pressure at the burner will cause a flame that ignites but can’t sustain. Check: