Viessmann [F.381] fault codewhat it means and how to fix it

F.381 — "Burner in a fault state — Flame loss during operating phase" — and F.378 — "Flame loss during stabilisation or operating phase" — are the lockout rows for a flame that was established and then went out. The measures are the flame-loss set: "Check gas supply (gas pressure and gas flow switch). Check balanced flue system for flue gas recirculation. Check ionisation electrode, burner gauze assembly: clearance to burner gauze assembly, contamination on electrode. Reset the appliance." The F.378 row phrases the middle item as "Check flue gas recirculation" and "Check for contamination of ionisation electrode and burner gauze assembly", which is the same instruction. The numeric table has a third row with the F.378 wording, F.64, whose system characteristic is "Normal operation; burner restarts" — so the same words can be a message or a fault depending on the number, and the number is what to write down.

Why a running burner loses its flame is a shorter list than why one fails to light. Gas that drops away under load; combustion air contaminated with exhaust; an electrode whose signal was marginal and finally fell below the threshold; and, on a modulating burner, a mixture at the low end of its range that a gust down the flue can blow out. Viessmann's combustion controller regulates the air ratio from the ionisation signal, so an electrode that is dirty or wrongly gapped does not just fail to see the flame, it steers the burner toward a mixture that is easier to lose. A flame that goes out repeatedly with the flue in good order is usually that electrode. The householder may reset once with the arrow keys after the burner has cooled and must notify the contractor if the fault recurs; nothing inside the case is user-serviceable.