Boiler shows E04, no ignition, fan may run, sometimes accompanied by gurgling sounds in the flue.
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.
Possible Causes
How to Fix / Troubleshooting
Safety first: Isolate electrical power before removing the boiler cover. Work on combustion air and flue components only if you are qualified.
User-level checks:
- Check that the external flue terminal is not blocked and that there is no visible damage to the flue.
- Attempt a single reset. If E04 reappears, a technician is required.
Technician checks:
- Inspect the air pressure switch and its silicone tubes for splits, blockages, or water; clear or replace as needed.
- Check that the fan starts and reaches the correct speed; measure supply voltage and feedback signal to the main PCB.
- Ensure the flue and air intake are correctly installed and not restricted.
- Test continuity and switching of the air pressure switch with a manometer or by simulating pressure; replace if defective.
Do not bridge the air pressure switch for testing while leaving the boiler in service; this defeats a critical safety device.
Repair Difficulty
Required Part
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