Viessmann [F9] fault codewhat it means and how to fix it

F9 reads "Burner in a fault state — Fan speed too low during burner start" on every two-character Vitodens unit from the WB1A to the B1HC and on the 050-W BPJD, and the Vitotronic-controlled 200-W, 222-F and 242-F carry the same row. Before ignition the burner control unit runs the fan up to its pre-purge speed and reads the speed back from the fan's own tacho signal; if the reported speed stays below what the start sequence needs, no gas is released and the sequence is abandoned. That is the intended behaviour of a fan-assisted premix burner — a MatriX burner with too little air would not light cleanly — so the code is more a refusal than a failure. Viessmann's measures are "Check the fan, fan connecting cables, fan power supply and fan control. Press Reset."

There is nothing here for the householder beyond the single reset the operating instructions allow (rotary selector to RESET under two seconds on WB1B, WB1C and B1HA; "R" for about two seconds on B1HC; the Reset key on a WB1A; MODE and OK on a BPJD; reset button R on a Vitotronic unit) and the usual instruction to call the contractor if it recurs. A fan that is slow to start is generally either starved of its supply — the 230 V feed and the PWM control signal both go through connectors that work loose — or mechanically tired; a bearing that drags at start-up and frees off warm produces an F9 on cold mornings. FA, the neighbouring code, is the opposite complaint: the fan would not stop. On the 2020-on numeric-table units F9 becomes F.71 (fan speed too low) and F.371, and F.457 is a fan that is sluggish or blocked.