Viessmann [b0] fault codewhat it means and how to fix it

The flue gas temperature sensor sits in the flue outlet of the heat exchanger and gives the control unit the reading behind F1 (maximum flue gas temperature exceeded). b0 is "Burner blocked — Short circuit, flue gas temperature sensor" and b8 is "Burner blocked — Lead break, flue gas temperature sensor", measure "Check the sensor", on the WB1B, WB1C, B1HA/B1KA and B1HC/B1KC, the 111-W B1LA to B1LD, the 050-W BPJD and the Vitotronic 200-W B2HA, 222-F and 242-F. The 2019 B2HB edition promotes both to "Burner in a fault state", which means a lockout needing the R key rather than an automatic restart; every other edition blocks the burner and releases it when the sensor returns. A boiler that cannot see its flue gas temperature cannot protect its plastic flue, so it does not run.

The WB1A table has no flue gas sensor rows at all. Because the sensor is in the flue gas path, Viessmann's repair chapter for the B1HC also gives a positioning check — code A3 is the sensor fitted in the wrong position — and a sensor that has been disturbed during a service can produce b8 before it produces A3. There is no householder step and no reset on the two-character units; on a Vitotronic B2HB the one permitted press of R applies. On the 2020-on numeric-table units the pair is F.49 (lead break) and F.50 (short circuit), both "Burner in a fault state", and F.393/F.394 report a second flue gas sensor where one is fitted.