Viessmann [F.67] fault codewhat it means and how to fix it

Four numbers in the numeric table carry the cause "Ionisation current lies outside the permissible range": F.67, F.69, and on the B1GA and 050-W B0HA/B0KA tables F.350 and F.351 as a pair. All are "Burner in a fault state". On these boilers the ionisation current is not only proof of flame but the signal the electronic combustion controller regulates air ratio from — Viessmann's appendix explains that the current peaks at an air ratio of 1 and the controller holds the burner between λ 1.2 and 1.5 by reading it — so a current the controller cannot make sense of is a combustion it cannot regulate, and it stops. F.67's measures start outside the burner: "Check gas supply (gas pressure and gas flow switch), check gas solenoid valve and inlet strainer", then the electrode's distance to the burner gauze and contamination of either, and finally "If specified measures don't help, replace fan unit." F.69's are more specific: whether the insulation block is touching the electrode ceramic, a gas-valve test at the lower heating output for about four minutes under service menu "b.6" (if the fault occurs, the HBMU heat management unit is replaced), then switching to the upper output — if the fault appears during modulation, the intake screen is checked for contamination and the fan unit replaced if necessary.

The fan unit appearing as a remedy for an electrode fault is not a misprint. On these Vitodens the gas/air mixture is formed in the fan unit's venturi and a contaminated intake screen or a worn fan skews the mixture, which the ionisation reading then reports as implausible. It is also why Viessmann's note about deposits on electrodes indicating foreign bodies in the combustion air matters: a boiler in a utility room full of tumble-dryer lint or laundry aerosols will show these codes long before a boiler in a clean cupboard. The householder's options are the single arrow-key reset and the call to the contractor if the code recurs.