Viessmann [F0] fault codewhat it means and how to fix it

F0 is the shortest row in the Viessmann table: "Burner blocked — Internal fault — Replace control unit." It appears with exactly those words on the WB1A, WB1B, WB1C, B1HA/B1KA and B1HC/B1KC, the 111-W B1LA to B1LD, the 050-W BPJD and the Vitotronic 200-W B2HA and 242-F B2UA. The burner control unit is the safety-relevant electronics that sequence the fan, gas train, ignition and flame monitoring; it runs continuous self-checks, and F0 is the code it raises when one of them fails in a way it cannot attribute to an external component. "Burner blocked" means the boiler stops and would retry if the condition cleared, but an internal fault rarely does, so in practice F0 is a boiler that is off until the unit is changed.

The 2019 B2HB edition is the one exception in tone: "Burner blocked or in a fault state — Internal fault — Press reset button R. Replace control unit if fault is permanent." So on a Vitotronic 200-W B2HB a single press of R is what Viessmann themselves suggest before condemning the unit; on every other unit the table offers no reset for F0 at all. Before a control unit is bought it is worth the contractor confirming the mains supply, because the WB1A table lists a separate E4 "Fault – supply voltage" and the B1HC lists 0C "Mains voltage too low", and a brown-out on a unit without those rows can present as F0. There is no householder step. On the 2020-on numeric-table units the equivalent is F.70 (HBMU heat management unit internal fault) and F.364.