The furnace indicates an external thermostat or control signal fault and will not respond to the room thermostat.

Safety: Isolate the power before working on control wiring.

• Check thermostat: Verify that the room thermostat or Baxi controller has power and is calling for heat.
• Inspect wiring: At the furnace terminal strip, check that the thermostat wires are secure and not damaged. Tighten any loose screws.
• Test by bridging: Temporarily fit a link across the room thermostat terminals at the furnace (as per the Baxi wiring diagram). If the furnace now responds, the external control or its wiring is faulty.
• Restore correct wiring: Remove the temporary link and repair or replace the thermostat/control as needed.