F.25
Vaillant Flue Gas Temperature Too High / Flue Gas Sensor Trip

Boiler locks out with F.25, fan may run but burner does not stay lit

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Blocked or restricted flue, Faulty flue gas temperature sensor, Recirculation of flue gases due to poor terminal location, Fan speed too low or fan failure

How to Fix / Troubleshooting

Critical safety: This fault can indicate dangerous flue gas conditions. Do not operate the boiler until the cause is identified. Turn off gas and power.

User-level checks:

  • Visually inspect the external flue terminal for obstructions (ice, leaves, nests, debris). Do not modify the flue.
  • Ensure no objects are blocking air intake or exhaust around the terminal.

Technician-level checks (for qualified personnel only):

  • Measure resistance of the flue gas temperature sensor and compare with Vaillant specifications; replace if out of range.
  • Inspect the flue system for correct assembly, slope, and integrity.
  • Check fan operation and speed feedback to the PCB.

Always use a Gas Safe registered engineer for flue and combustion-related work.

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Repair Difficulty

Professional Required 5/5

Required Part

Flue Gas Temperature Sensor
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