BoilerFaultCodesVaillant F.77 Fault Code
Also written as F77 — same fault.
The Vaillant F.77 fault code means: your boiler is struggling to remove excess water or is experiencing a communication error with an external safety part, causing it to shut down to prevent damage.. It appears on 29 Vaillant models in our database.
Technical description: Condensate pump or return signal from accessory module heating: Condensate pump faulty or return signal from the exhaust gas flap has responded
What causes the Vaillant F.77 fault?
This fault occurs when the boiler's safety system detects that the pump responsible for removing waste water is broken or a specific safety flap is stuck. If the condensate water cannot be pumped away, it builds up inside the unit, forcing the boiler to shut down to prevent internal flooding or hazardous gases from escaping. It is essentially a safety 'cut-off' triggered by an issue with an add-on component or accessory module.
This fault is more common during freezing winter temperatures, as external pipes can freeze and cause water to back up, putting extra strain on the pump until it fails.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like F.77: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Vaillant models that show the F.77 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412 Repair: £150-300
- ecoFIT Pure 418 Repair: £150-350
- ecoFIT Pure 425 Repair: £150-350
- ecoFIT Pure 435 Repair: £150-300
- ecoFIT pure 612 Repair: £150-300
- ecoFIT pure 615 Repair: £150-300
- ecoFIT pure 618 Repair: £150-300
- ecoFIT pure 630 Repair: £150-300
- ecoFIT Pure 825 Repair: £150-300
- ecoFIT Pure 830 Repair: £150-300
- ecoFIT Pure 835 Repair: £150-350
- ecoFIT sustain 415 Repair: £150-300
- ecoFIT sustain 418 Repair: £150-300
- ecoFIT sustain 430 Repair: £150-300
- ecoFIT sustain 615 Repair: £150-300
- ecoFIT sustain 618 Repair: £150-300
- ecoFIT sustain 630 Repair: £150-300
- ecoFIT sustain 825 Repair: £150-300
- ecoFIT sustain 830 Repair: £150-300
- ecoFIT sustain 835 Repair: £150-300
- ecoTEC exclusive Repair: £120-280
- ecoTEC plus Repair: £120-280
- ecoTEC plus VU GB 656/4-5 A Repair: £150-350
- ecoTEC pro Repair: £120-280
- ecoTEC sustain Repair: £120-350
- Home Combi 25 Repair: £150-300
- Home Combi 30 Repair: £150-300
- Home Combi 35 Repair: £150-300
- Home Regular 12
Source
Vaillant F.77 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
- Vaillant ecoFIT Pure 425 manual (PDF)
- Vaillant ecoFIT Pure 435 manual (PDF)
- Vaillant ecoFIT pure 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026