U1
Buderus
Supply Voltage Fault / Under- or Overvoltage
U1 code, furnace unresponsive or randomly resets
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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
Low line voltage, Loose power connections, Faulty main power switch, Internal power supply failure on Buderus control board
How to Fix / Troubleshooting
Safety: Turn off power at the breaker before inspecting electrical connections.
Steps:
- Check supply voltage: Have a qualified person measure voltage at the furnace terminals. It should match the rating plate (typically 120 V or 230 V).
- Inspect wiring: Tighten any loose terminal screws at the service switch and furnace junction box. Look for burned or discolored wires.
- Dedicated circuit: Ensure the furnace is on a dedicated circuit and not sharing with large appliances that cause voltage dips.
If U1 persists with correct voltage, the internal power supply on the Buderus control board may be failing and should be replaced by a technician.
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Repair Difficulty
Hard
4/5
Required Part
Buderus Control Board (Main PCB)
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