A3
Worcester Bosch
Fan Fault / Air Proving Failure
Boiler shows A3, often with the fan not running or running erratically.
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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
Failed combustion fan, Blocked flue or air intake, Faulty fan speed sensor, PCB not supplying power to fan
How to Fix / Troubleshooting
Safety first: Isolate power and gas. The fan is part of the combustion circuit; only a Gas Safe engineer should service it.
- Check flue terminal: From outside, ensure the Worcester Bosch flue terminal is not obstructed by debris, bird guards, or ice.
- Listen for fan operation: When calling for heat, you should hear the fan start before ignition. If silent, the fan or PCB may be faulty.
- Engineer tasks: A technician should test the fan for correct voltage, check the air pressure switch or differential sensor (if fitted), inspect the flue for blockages, and replace the fan assembly if seized or noisy.
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Repair Difficulty
Professional Required
5/5
Required Part
Combustion Fan Assembly
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