High
F.77 on Vaillant ecoFIT Pure 412

Your boiler is struggling to clear away liquid waste or detected a blockage in the exhaust system, which has caused it to stop working as a safety precaution.

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

Condensate pump or flue gas non-return flap fault

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.

Find a Gas Safe engineer
Search the official register →

Related fault codes

F.22View →F.28View →F.29View →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

Frozen condensate pipe

Winter cause of ignition lockouts.

Read →