C0
Viessmann
No Heat Source Detected on Bus
Thermostat shows C0 and cannot detect any boiler or heat source on the KM‑BUS; system appears offline.
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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
Boiler not powered or in standby, KM‑BUS cable disconnected at boiler, Wrong bus type selected in thermostat, Faulty boiler main control board
How to Fix / Troubleshooting
Safety first: Turn off power before touching wiring. Do not open the boiler’s sealed combustion area.
- Check boiler power: Ensure the boiler’s main switch is on and that its display is lit. Check the circuit breaker or fuse if it is off.
- Inspect KM‑BUS connection at boiler: Verify that the KM‑BUS cable from the thermostat is connected to the correct bus terminals on the boiler control board.
- Confirm bus type in thermostat: In installer settings, ensure the thermostat is configured for KM‑BUS communication with a Viessmann boiler, not for on/off relay control.
- Test with another device: If available, connect another known‑good Viessmann room unit to the bus to see if it detects the boiler.
- Call technician: If the boiler is powered but no bus communication is possible, the boiler control board may be defective and requires professional replacement.
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Repair Difficulty
Hard
4/5
Required Part
Boiler main control board (KM‑BUS interface)
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