Room thermostat or ATAG controller calls for heat but furnace does not respond

Safety first: Isolate power before working on control wiring.

• Check that the room thermostat or controller has power (display on, batteries good if wireless).
• Inspect the low-voltage control wiring at the boiler terminals for loose or broken conductors.
• Verify that the control mode (e.g., on/off, OpenTherm, ATAG bus) is correctly configured in the boiler settings according to the controller type.
• If wiring and settings are correct, try temporarily bridging the call-for-heat terminals (if safe and per manual) to see if the boiler responds; if it does, the thermostat is likely faulty.