F.49
Vaillant
Low Voltage / 24V DC Supply Fault
Boiler shows F.49, controls unresponsive or intermittent
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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
Short circuit on external controls (eBUS, room thermostat), Faulty external control wiring, Overloaded PCB low-voltage supply, Defective PCB
How to Fix / Troubleshooting
Safety: Isolate power before working on control wiring.
- Power cycle: Turn off the boiler at the fused spur for 2–3 minutes, then turn back on.
- Disconnect external controls: Remove eBUS or thermostat connections at the boiler terminal strip (note wiring first). Power the boiler and see if F.49 clears. If it does, the fault lies in external wiring or controls.
- Inspect cables: Check for damaged or crushed thermostat/eBUS cables, especially where they pass through walls.
- Test with basic link: Temporarily fit a link on the room thermostat terminals (if applicable) to simulate a call for heat. If the boiler works, replace the faulty control.
- PCB replacement: If F.49 remains with all external controls disconnected, the PCB low-voltage supply may be faulty and needs professional replacement.
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Repair Difficulty
Medium
3/5
Required Part
Main Control PCB
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