Viessmann Error Code F.403: IO electrode ground fault (Boiler)
Viessmann error code F.403 on a boiler means: IO electrode ground fault. It is also called Viessmann diagnostic code F.403.
Homeowner? What Viessmann code F.403 means for you
Call a technician Your boiler is reporting: IO electrode ground fault. The boiler has locked out or cannot run properly. One reset is fine; if the code comes back, it needs a licensed technician to find the cause.
Safe things you can check yourself
- Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
- Make sure the unit has power: its on/off switch (often looks like a light switch near the unit) is ON and the breaker in the electrical panel is not tripped.
- Look at the pressure gauge on the boiler. If it reads much lower than usual or near zero, or water is leaking, call a technician.
- Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
- Check that the boiler gas valve is open (handle in line with the pipe).
- Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.
Call (647) 657-2977Book a service call
Licensed technicians serving Vaughan, Toronto and the GTA.
What does Viessmann error code F.403 mean?
Burner in a fault state; ground fault on ignition/ionization electrode.
Applies to: Vitodens 200-W B2HE 85, 120, 150, 199
Common causes of Viessmann code F.403
- Electrode ground fault
- Faulty BCU
Viessmann models that use code F.403
B2HE-85B2HE-120B2HE-150B2HE-199
Other Viessmann Boiler codes
All Viessmann Boiler error codes →
Viessmann code F.403: homeowner questions
What does Viessmann code F.403 mean?
IO electrode ground fault. Burner in a fault state; ground fault on ignition/ionization electrode.
Is it safe to keep using my boiler with code F.403?
It depends on the cause. The boiler has locked out or cannot run properly. One reset is fine; if the code comes back, it needs a licensed technician to find the cause.
Can I reset Viessmann code F.403 myself?
You can try one reset (power off for 30 seconds, then on). If Viessmann code F.403 comes back, the cause is still there and a licensed technician should check it.
Who can fix Viessmann code F.403 near me?
Aurora Star Tech Inc. repairs Viessmann boilers in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.
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Troubleshooting guides
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- Where the fault history lives Viessmann · Boiler
Source: Viessmann Vitodens 200-W B2HE 85/120/150/199 Service Instructions 6167 813-24 (09/2026), Fault messages, p.62-73. Written in our own words as a summary; always check the original manual.
General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.