Viessmann [E5] fault codewhat it means and how to fix it

E5 is the row whose cause column says least and whose measures column says most. On every two-character Vitodens control unit — WB1A, WB1B, WB1C, B1HA/B1KA, B1HC/B1KC, the 111-W B1LA to B1LD and the 050-W BPJD — the system characteristic is "Burner blocked", the cause is "Internal fault", and the measure is "Check the ionisation electrode and connecting cables. Press Reset." That measure tells you what the internal fault is: the burner control unit has found the flame-monitoring circuit reading something it cannot reconcile, most often an ionisation current present or absent at a moment in the sequence when the opposite was required, and rather than release gas on a signal it does not trust it blocks the burner. Blocked rather than locked out matters. Viessmann's "Burner blocked" rows describe a boiler that will try again on its own when the condition clears, and E5 frequently does clear, then returns, because a deteriorating electrode gives a marginal signal that passes one cycle and fails the next.

The reset in the measures column is for the contractor after the electrode and cables have been checked. There is nothing in Viessmann's operating instructions for the householder to do with E5 beyond noting the code, and because the component behind it is the one that proves a flame exists, this page carries the carbon monoxide flag.

E5 means something different on a Vitotronic-controlled boiler. The 200-W B2HA, 222-F B2TA and 242-F B2UA table reads "Burner blocked — Flame amplifier fault — Replace control unit"; the 2019 B2HB edition softens it to "Press reset button R. Replace control unit if fault is permanent." Same component family — the flame amplifier is the circuit the ionisation electrode feeds — but on the Vitotronic units Viessmann skip the electrode check and go straight to the control unit. On the 2020-on numeric-table units the closest rows are F.67 and F.69 (ionisation current lies outside the permissible range).