Furnace shows F1, no response to thermostat demand

Safety first: Isolate power before working on control wiring.

• 1. Check thermostat wiring: Inspect the low-voltage wires between the furnace and the room thermostat or Junkers controller (e.g., FR, FW series). Look for breaks, loose terminals, or short circuits.
• 2. Bypass test: Temporarily bridge the thermostat terminals at the furnace (consult wiring diagram). If the furnace starts, the issue is with the thermostat or its wiring.
• 3. Controller configuration: For digital Junkers controllers using BUS communication, ensure they are correctly addressed and configured according to the installation manual.
• 4. Replace faulty controller: If wiring is sound but communication fails, replace the room controller with a compatible Junkers model.
• 5. Restore proper wiring: Remove any temporary bridges and reconnect the thermostat correctly before returning the furnace to service.