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

Fd and FF share a row on the two-character Vitodens units. On the WB1A, WB1B, WB1C, B1HA/B1KA and B1HC/B1KC, the 111-W B1LA to B1LD and the 050-W BPJD both read "Burner blocked — Burner control unit fault — Check ignition electrodes and connecting cables. Check whether a strong interference (EMC) field exists near the appliance. Press Reset. Replace control unit if fault persists." The order of those measures is the useful part. The ignition electrodes sit millimetres from the burner and are fed several kilovolts by the ignition module; a cracked insulator or a lead routed against the casing lets that energy arc somewhere it should not, and the burner control unit's supervision sees the disturbance as a fault in itself. Hence the EMC line — Viessmann are telling the contractor that a healthy control unit can throw Fd because of what is happening around it.

On the Vitotronic 200-W B2HA and 242-F B2UA Fd means something else: "Burner in a fault state, and additional fault b7 is displayed — Coding card missing — Insert coding card. Press reset button R." The coding card is the small plug-in module that tells a Vitotronic control unit which boiler it is fitted to; without it the unit cannot run. The 2019 B2HB edition returns to the 100-W meaning and adds "If the fault occurs sporadically, replace the coding card." FF on a Vitotronic unit is different again — "Internal fault or reset button R disabled — Restart the appliance" — and has its own page. For the householder, Fd on any unit takes the single reset for that unit and then the contractor. On the 2020-on numeric-table units the closest rows are F.70 and F.77 (HBMU internal fault and data memory).