Vaillant [F.25] fault codewhat it means and how to fix it

F.25 is the clearest case of a Vaillant code changing its meaning across generations, so start with which boiler you have. On the ecoTEC plus, pro and exclusive, the ecoFIT pure and sustain and the open-vent ecoTEC plus, and as F.025 on the 2022 CS/1-5, it is "safety switch-off: flue gas temperature too high"; the ecoTEC sustain table does not carry it.

The boilers can be fitted with an optional flue gas safety cut-out, and the ecoTEC plus check list runs through the plug and cable harness for it, the cable to the heating pump, the internal air vent, the flue route for blockage, adverse wind or an over-long flue, insufficient water in the heating circuit and the pump — so on those boilers F.25 is mostly a circulation code in disguise, because flue gas runs hot when the heat exchanger is not carrying heat away. The ecoFIT and exclusive tables add the electrical version: a break in the plug connection for the optional cut-out, or in its harness.

On the 2009 ecoTEC plus and pro the same number is "interruption in the compact thermal module cable harness", a wiring fault on the heat cell, and on the ecoMAX 600 and 800 series it is "water switch — plug disconnected, system pressure too low, water switch activated", a low-water protection. Three meanings, one code; the page below follows the current one.