B3
Viessmann
Boiler Lockout Reported to Thermostat
Thermostat displays B3 and reports that the boiler is in lockout; no heating despite 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
Boiler safety lockout due to flame failure or overheat, Low system pressure detected by boiler, Faulty boiler sensor causing lockout, Gas supply interruption
How to Fix / Troubleshooting
Safety first: Do not attempt to open or repair the boiler combustion chamber or gas components. Gas work must be done by a licensed professional.
- Check boiler display: Go to the boiler and read the error code shown on its display. Follow the user instructions for resetting if allowed.
- Verify system pressure: Ensure the heating system pressure is within the recommended range (typically around 1.0–1.5 bar when cold). Top up via the filling loop if the manual permits and you are comfortable doing so.
- Reset boiler: Use the boiler’s reset button as described in the manual. Do not repeatedly reset a boiler that immediately locks out again.
- Check gas supply: Confirm that other gas appliances (if present) are working and that the gas supply has not been shut off.
- Call a technician: If the boiler will not reset or repeatedly locks out, contact a Viessmann‑approved heating engineer to diagnose and repair the boiler fault.
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Repair Difficulty
Professional Required
5/5
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