What does the Vaillant F.77 fault code mean?

Your boiler is struggling to push waste gases out through the flue or is unable to clear internal condensation water properly.

How much does it cost to fix Vaillant error code F.77?

Typical repair cost for the Vaillant F.77 fault is £120-350 including labour.

Vaillant F.77 Fault Code: What It Means & How to Fix It

What the manual says

Manufacturer manual

Condensate pump or feedback of accessories blocks heating | Condensate pump defective or flume flap feedback triggered

Shared across the range

This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.

What causes this fault?

This fault occurs when the boiler detects a blockage or failure in the system that removes waste gases and water. It is usually caused by a stuck internal flap that prevents exhaust fumes from escaping safely, or a pump failure that stops waste water from being drained away. When these systems fail, the boiler shuts down as a safety precaution to prevent internal flooding or the buildup of dangerous gases.

This fault can be more frequent during extreme cold spells if the external pipe that carries waste water away freezes and causes a backup into the pump area.

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

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Related fault codes

F.20View →F.73View →F.74View →

All 170 fault codes on the ecoTEC Plus 625

Seeing a different code? Every code this model can display, in one place.

con d.00 D.000 d.01 d.02 d.04 d.05 d.06 d.07 d.08 d.09 d.10 d.11 d.13 d.14 d.15 d.16 d.17 d.18 d.19 d.20 d.21 d.22 d.23 d.24 d.25 d.27 d.28 d.31 d.33 d.34 d.35 d.36 d.39 d.40 d.41 d.43 d.45 d.47 d.50 d.51 d.58 d.60 d.61 d.62 d.64 d.65 d.66 d.67 d.68 d.69 d.70 F.00 F.0 F.01 F.1 F.10 F.11 F.12 F.13 F.20 F.22 F.23 F.24 F.25 F.27 F.28 F.29 F.32 F.33 F.49 F.61 F.62 F.63 F.64 F.65 F.67 F.68 F.70 F.71 F.72 F.73 F.74 F.75 F.76 F.78 F.83 F.84 F.85 F.86 F.87 F.88 F.89 F.91 F.92 F.97 F22 F70 P.00 P.0 P.01 P.1 P.02 P.2 P.3 P.4 P.5 P.06 P.6 P.07 S.0 S.01 S.1 S.02 S.2 S.03 S.3 S.04 S.4 S.05 S.5 S.06 S.6 S.07 S.7 S.08 S.8 S.10 S.11 S.13 S.14 S.15 S.16 S.17 S.20 S.21 S.22 S.23 S.24 S.25 S.26 S.27 S.28 S.30 S.31 S.32 S.33 S.34 S.36 S.39 S.41 S.42 S.46 S.53 S.54 S.88 S.91 S.96 S.97 S.98 S.99 S.108 S.109 T.02 T.03 T.04 T.05 T.06 T.08

Your boiler is struggling to push waste gases out through the flue or is unable to clear internal condensation water properly.

What the manual says

What causes this fault?

Engineer required

Avoid the next call-out bill

Related fault codes

All 170 fault codes on the ecoTEC Plus 625

Related guides you might find useful

Banging / kettling / whistling noise

Yellow or orange flame — CO safety

Frozen condensate pipe

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