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 any external smart thermostats or zone controllers for low batteries or error messagesEnsure all external timers and programmers are set to the same heating modePerform a simple reset of the boiler using the power button to clear the communication glitch