B0
Viessmann
Intermittent KM‑BUS Communication Error
Thermostat shows B0 and intermittently loses connection to the boiler; heating output is sporadic.
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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
Poor contact in KM‑BUS terminals, Electrical noise on KM‑BUS cable due to parallel mains routing, Excessive KM‑BUS cable length or incorrect cable type, Partially broken conductor in bus cable
How to Fix / Troubleshooting
Safety first: Switch off power to the boiler and thermostat before working on wiring.
- Re‑terminate KM‑BUS: Remove and reinsert the KM‑BUS wires at both thermostat and boiler ends, trimming back any oxidized copper and ensuring a clean, tight connection.
- Check cable routing: Ensure the KM‑BUS cable is not run in parallel with mains cables over long distances. If it is, consider rerouting or using shielded cable as recommended by Viessmann.
- Inspect for mechanical damage: Gently flex the cable along its length; if communication drops when the cable is moved, a conductor may be broken internally. Replace the cable.
- Limit bus length and devices: Verify that the total bus length and number of devices on the KM‑BUS are within Viessmann specifications.
- Test after power cycle: Restore power and monitor for stability. If B0 persists, a technician should check the boiler’s KM‑BUS interface.
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Repair Difficulty
Medium
3/5
Required Part
KM‑BUS communication cable
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