Viessmann [F.68] fault codewhat it means and how to fix it

F.68 — "Burner in a fault state — Flame signal is already present at burner start." — is the control unit refusing to open the gas valve because its flame detector says a flame already exists. A real flame with the valve shut would mean the valve is leaking, so the unit treats the signal as true and stops. In practice the signal is almost always false: the ionisation electrode circuit has found a path to earth, or moisture is bridging it, or the heat management unit's input has failed. Viessmann's measures are written to separate those: "Close the gas shut-off valve. Remove connecting cable of the ionisation electrode. Reset the appliance. If the fault persists, replace the HBMU heat management unit." With the electrode disconnected there can be no genuine signal, so a persisting F.68 is the unit itself. F.355, "Analogue signal reference check: Flame signal is already present at burner start", is the same conclusion reached by the unit's own self-test, and its only measure is to replace the HBMU.

The instruction to close the gas valve first is there for the one case that is not electrical, and it is why this is not a code to reset repeatedly. The householder can do the one thing the operating instructions allow for any lockout — a single reset with the arrow keys — and if F.68 returns should leave the boiler off and call the contractor. On the two-character units the same condition has no dedicated row, and on the Vitotronic 200-W it appears as EF or a flame-monitoring fault in words; the numeric table is the only one that names it this clearly.