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F.403

F.403 — Burner in fault state due to ionization electrode ground fault

Viessmann · Vitodens 100-W Boiler

What does F.403 mean?

This fault indicates that the burner is in a fault state, likely caused by a ground fault with the ionization electrode or a problem with the BCU burner control unit. The boiler's operation is halted.

Common Causes

  • Ionization electrode ground fault
  • BCU burner control unit is faulty

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Check the IO electrode for any signs of a ground fault or damage.

  2. 2

    Reset the boiler by briefly turning off and on its power supply.

  3. 3

    If the fault persists, contact a qualified heating engineer, as the BCU burner control unit may need to be replaced.

Browse all codes in this manual (60)
Verified technical data. Last updated: April 2026

Related Faults

Connection errorConnection error — Displayed on programming unitF.142F.142 — Burner in fault state due to CAN bus communication errorF.160F.160 — Burner in fault state due to CAN bus communication error
Source: Viessmann Vitodens 100-W Boiler
Open Manual (Page 79)Search more codes in this manual