Viessmann [51] fault codewhat it means and how to fix it

A Vitodens combi heats tap water through a plate heat exchanger and steers the burner by an outlet temperature sensor on the hot side of it. 51 is "No DHW heating — Short circuit, outlet temperature sensor" and 59 is "No DHW heating — Lead break, outlet temperature sensor", measure "Check the sensor", on the WB1A, WB1B, WB1C, B1HA/B1KA and B1HC/B1KC combis, the 111-W, the 050-W BPJD and the Vitotronic 200-W B2HB and 242-F B2UA. Without a plausible outlet reading the control unit will not run a hot-water cycle, so the symptom is a boiler that heats radiators normally and does nothing when a tap is opened; central heating is untouched. The WB1A table prints "short circuit" for both 51 and 59, which is a misprint carried in that edition — every later table gives 59 as the lead break.

Viessmann's repair note for this sensor is the reason the job is a contractor's: water can leak out when the outlet sensor is replaced, and the cold water shut-off valve must be closed and the DHW pipe and plate heat exchanger drained on the secondary side first. There is no householder step and no reset; the code clears itself when the sensor reads plausibly. On the 2020-on numeric-table units the pair is F.182 (short circuit) and F.183 (lead break), flagged "if installed"; a bare 59 on that display is F.59, mains undervoltage, which is unrelated and has its own page.