F12
Viessmann
Communication Fault with Room Controller / Bus
Boiler shows F12, no response to thermostat demand
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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
Broken KM-BUS / OpenTherm cable, incorrect polarity on bus terminals, incompatible controller, faulty control board communication circuit
How to Fix / Troubleshooting
Safety: Isolate power before working on control wiring.
- Check controller power: Ensure the Viessmann room controller (e.g., Vitotrol) powers up and displays correctly.
- Inspect bus wiring: Verify the two-core bus cable is connected to the correct terminals on both boiler and controller, with no shorts or breaks.
- Polarity: Some Viessmann bus systems are polarity-sensitive. Match + and - as per the installation manual.
- Test with link: Temporarily disconnect the controller and fit a link (if specified by Viessmann) to simulate demand. If the boiler runs, the controller or wiring is suspect.
- Controller replacement: If wiring is sound, try a known-good controller or have the existing one tested.
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Repair Difficulty
Medium
3/5
Required Part
Room Controller / KM-BUS Interface
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