BoilerFaultCodesTechnical description
External heating program changeover applies to heating circuit with mixer
What causes this fault?
This fault occurs when the boiler’s main control board loses the electronic signal from the module that manages different heating zones or underfloor heating. It is usually caused by a loose connection in the wiring between these components or a fault within the external mixer module itself, which prevents the boiler from knowing which temperature to send to specific parts of your home.
This issue is often noticed in autumn when homeowners switch their heating back on after summer and the external control modules fail to synchronise correctly.
DIY fix possible
Check your external thermostat or wall controllers for any unusual settings or schedulesTurn the boiler off and back on at the fused spur switch to reset the communication linkVerify that any smart home apps connected to the heating are not sending constant override commands