The furnace display works, but there is no response to the room thermostat, and heating does not start.

Safety first: Isolate power before working on control wiring.

• 1. Check thermostat settings: Ensure the room thermostat is set above room temperature and in heating mode.
• 2. Wiring inspection: Verify that thermostat wires are correctly connected to the designated Junkers terminals (e.g., TA, BUS) according to the wiring diagram.
• 3. Bypass test: Temporarily bridge the thermostat contacts at the boiler. If the boiler starts, the thermostat or its wiring is faulty.
• 4. Replace thermostat: Install a compatible Junkers or approved thermostat if the existing one is defective.