BoilerFaultCodesTechnical description
External heating program changeover applies to heating circuit without mixer and heating circuit with mixer
What causes this fault?
This fault occurs when the boiler receives two different instructions at the same time from your external heating controls, such as a wall thermostat or a zone controller. It essentially creates a logic 'argument' within the system because the boiler doesn't know which setting to prioritize for your radiators. The most common reason is a wiring error or a faulty external programmer sending overlapping signals to the control board.
DIY fix possible
Check your external thermostat or heating app for conflicting schedule settingsEnsure all external controls are calling for heat correctlyReset the boiler using the power button to clear temporary communication glitches