Viessmann [F5] fault codewhat it means and how to fix it

F5 is a fan fault on the Vitodens range. The control board commands a fan speed and reads back the actual speed; when the two disagree it will not permit the burner to fire, because it cannot confirm that combustion air is being supplied and flue gases cleared. Worn bearings are the usual physical cause and they generally announce themselves with weeks of noise beforehand. The alternative is a restriction in the flue or air intake, which produces the same reading but carries a genuine combustion risk, so a competent engineer will check the flue rather than simply fitting a fan and leaving.

1. 1

   Check the flue terminal outside for nests, debris, or anything newly built close to it.

   Look only — never push anything into a flue terminal.
2. 2

   Listen at startup and note any rattle, drone or grinding to tell the engineer.

3. 3

   Reset once, and stop if it returns.