F.25 — Safety switch-off: Flue gas temperature too high

What does F.25 mean?

The boiler has shut down because the flue gas temperature has exceeded safe limits. This is a safety measure to prevent overheating of the flue system.

• Break in the plug connection for the optional flue gas safety thermostat (STB).
• Break or damage in the cable harness connected to the flue gas thermostat.

Click steps to track your progress.

1. 1

   Press the fault clearance key on the boiler to reset it.

2. 2

   Visually inspect any accessible connections related to the flue gas system for dislodgement.

3. 3

   If the fault persists, contact Vaillant Service Solutions, as this is a critical safety issue.