Viessmann [F.369] fault codewhat it means and how to fix it

F.369 and F.380 share their cause word for word: "Flame is lost immediately after flame formation (during safety time)." Both are "Burner in a fault state". The safety time is the few seconds after ignition during which the control unit will tolerate an unstable flame before it must decide; a flame that appears and dies inside that window is a burner that lit on something — gas at the wrong pressure, air with flue gas in it, a mixture the electrode could barely see — and could not hold. The measures are the same for both numbers: "Check gas supply (gas pressure and gas flow switch). Check balanced flue system for flue gas recirculation. Check ionisation electrode: clearance to burner gauze assembly, contamination on electrode." F.369 ends "Reset the appliance"; the F.380 row does not print the line but the reset is the same.

The flue gas recirculation check is the one to take seriously. A balanced flue whose inner pipe leaks into the outer, or whose terminal is positioned so that exhaust is drawn back into the air intake, feeds the burner its own products; the flame lights and then starves. Viessmann put that check second, before the electrode, on every flame-loss row in the table. Gas flow pressure that collapses when the valve opens — a sticking regulator, an undersized or partly blocked supply — is the first. The electrode's gap to the burner gauze is a precise setting and the reason a cleaned-and-refitted electrode can still fail. The householder's part is a single reset and a call if it recurs; there is nothing to adjust from outside the case. F.381 (and F.378) is the same loss later, once the burner was established; F.64 is the version the boiler recovers from by itself.