Viessmann [F3] fault codewhat it means and how to fix it

F3 reads "Burner in a fault state — Flame signal is already present at burner start" on every two-character Vitodens control unit, the WB1A included. It is the inverse of F4: rather than no flame when one was expected, the flame-monitoring circuit reports a flame at the moment the burner control unit is about to start the sequence, before the gas train has been opened. A control unit that cannot trust its own flame signal cannot safely release gas, so it locks out. Viessmann's measure is short — "Check ionisation electrode and connecting cable" — because in practice a false flame signal is almost always the electrode or its lead leaking to earth, or a contaminated electrode conducting when it should not. A gas train genuinely passing gas with the burner off is the rarer and more serious reading of the same code, which is why this page carries the carbon monoxide flag and no householder steps.

The reset is the one for your unit — rotary selector to RESET for under two seconds on WB1B, WB1C and B1HA/B1KA; "R" touched for about two seconds on B1HC/B1KC; the Reset key on a WB1A; MODE and OK on a 050-W BPJD — and Viessmann allow one attempt before the contractor is called. If F3 comes back, do not keep resetting it. F3 keeps the same meaning on the Vitotronic-controlled 200-W, 222-F and 242-F, where the reset is the "R" key on the control unit and the B2HB edition adds "check system for condensate backup"; on the 2020-on numeric-table units the equivalent row is F.68 (flame signal is already present at burner start), which has its own page.