High Lockout
F.25 on Vaillant ecoTEC Pro 28

Your boiler has shut down automatically because it detected that the exhaust gases are getting too hot, which is a safety feature to prevent damage to the internal components.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Safety switch-off: Flue temp. too high - flue gas temperature too high

What causes this fault?

This fault occurs when the internal sensors detect that the exhaust gases are reaching dangerous temperatures that could melt or damage the boiler's components. The most common reason is a heavy buildup of limescale or debris inside the main heat exchanger, which prevents heat from being transferred to the water and pushes it out through the flue instead.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F.20View →F.75View →F.22View →

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