Boiler shows E12; heating demand not recognised from room or cylinder controls.

Safety: Isolate power before working on control wiring.

• Check control settings: On the CTC boiler control panel, ensure that the operating mode allows external thermostat control and that schedules are correctly set.
• Bypass test (technician): At the boiler's room thermostat terminals, temporarily link the two control terminals (as per CTC wiring diagram). Restore power. If the boiler now responds, the external thermostat or its wiring is faulty.
• Inspect wiring: Check the cable from the room thermostat or CTC controller to the boiler for damage, loose connections, or water ingress. Repair or replace as needed.
• Controller check: If using a CTC-branded controller, follow its diagnostic procedure or substitute a known-good unit.