Boiler fails to respond to room thermostat or external control signals; heating does not start even though there is demand.

Safety first: Turn off power before working on control wiring.

• Check thermostat power: Ensure the room thermostat or controller has power and displays correctly.
• Inspect wiring: Verify that thermostat wires are securely connected at both the thermostat and boiler terminals, with no shorts or breaks.
• Bypass test (technician): A professional can temporarily bridge the call-for-heat terminals at the boiler to confirm whether the issue is with the thermostat or boiler.
• Component replacement: Replace faulty thermostats or repair wiring as needed. If the boiler still does not respond, the PCB input may be defective.