A77
Vokera
Flue gas sensor / safety device fault reported by boiler
Thermostat displays A77 and heating is unavailable
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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
Faulty flue gas sensor in boiler, Blocked or restricted flue, Boiler PCB fault, Overheating in flue path triggering safety lockout
How to Fix / Troubleshooting
Safety first: Flue and combustion safety devices are critical. Do not attempt to dismantle the flue or bypass any safety sensor. If you suspect flue blockage, stop using the boiler and call a qualified engineer immediately.
- Check boiler display: Confirm that the boiler is showing A77 or a similar flue sensor error. The thermostat is mirroring this status.
- Visual flue check: From outside, ensure the flue terminal is not blocked by debris, snow, or vegetation. Do not insert objects into the flue.
- Reset and test: Reset the boiler once. If the error returns, do not continue resetting.
- Contact engineer: A Gas Safe engineer must inspect the flue system and boiler safety sensors. The thermostat does not cause this error.
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Repair Difficulty
Professional Required
5/5
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