Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
F.12 is the short-circuit counterpart of F.02: the actoSTOR charging NTC (or its cable) is shorted to itself or to earth. Only valid with F.91.
Vaillant meaning: Short circuit at cylinder charging sensor (NTC) — only in combination with F.91
Possible causes
- Charging NTC internally shorted
- Cable trapped, chafed, or shorted to the casing
- Water in a connector
Possible solutions
- Unplug the charging NTC. If F.12 clears, the sensor or its lead is shorted.
- Inspect the lead along the case and heat exchanger. Replace damaged insulation rather than taping it as a long-term fix.
- Replace the NTC, then the lead, then actoSTOR electronics if the short remains on the board input.
Possible parts needed
- actoSTOR charging NTC
- NTC cable / harness
- actoSTOR electronics module