F49
Viessmann
Low Voltage / Bus Short Circuit
Boiler shows F49, communication or low-voltage error, may affect controls
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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
Shorted external control wiring, faulty room thermostat, incorrect connection of 230 V to low-voltage terminals, internal PCB fault
How to Fix / Troubleshooting
Safety: Mixing mains and low-voltage circuits can destroy electronics and be hazardous.
- Disconnect externals: Isolate power, then disconnect all external control wiring (room stats, timers, bus devices). Re-energize and see if F49 clears.
- Reconnect one by one: Reconnect each external device in turn to identify which causes the fault.
- Inspect wiring: Ensure no 230 V is connected to low-voltage bus or thermostat terminals.
- Replace faulty device: Replace any thermostat or controller that causes F49 when connected.
- PCB check: If F49 remains with all externals disconnected, the PCB low-voltage section may be damaged and require replacement.
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Repair Difficulty
Hard
4/5
Required Part
Main Control Board
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