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

FC is one of the codes where the control unit decides the meaning. On the two-character 100-W units (WB1A excepted — its table has no FC), the 111-W B1LA to B1LD and the 050-W BPJD the row is "Burner blocked — Electrical fan control (control unit) faulty — Check fan connecting cable; replace if necessary or replace control unit." The burner control unit drives the fan with a PWM signal and reads its speed back; when that loop fails in a way the unit attributes to itself rather than to the fan, it blocks the burner. Blocked, not locked out: the boiler will try again by itself, and an FC that comes and goes is a fan lead with an intermittent connection, which is why Viessmann put the cable first.

On the Vitotronic-controlled 200-W B2HA and B2HB, 222-F B2TA and 242-F B2UA the FC row is "Burner in a fault state — Gas train faulty, faulty modulation valve control or flue gas path blocked — Check connecting cables to the gas train for damage. Check gas train. Check flue system for blockages or constrictions. Press reset button R." That is a lockout, on a different component, with a flue check in it — which is why this page carries the carbon monoxide flag even though the 100-W reading of the code does not need one. If your boiler prints a sentence beside the code, you have the second meaning. Neither version gives the householder anything to do; the 100-W reading needs no reset, and the Vitotronic reading takes the one press of R the operating instructions allow. On the 2020-on numeric-table units the fan-control fault is folded into the HBMU internal-fault rows (F.70 and neighbours) and the gas-train faults are F.348 and F.354.