Boiler displays A8 and does not respond to room thermostat commands

Safety: Turn off power before working on control wiring.

• Check controller display: Verify that the Junkers room thermostat or controller powers up and shows no own error codes.
• Inspect wiring: Remove the boiler front cover and check the low-voltage bus terminals (e.g., EMS/Heatronic). Ensure the polarity and terminal positions match the Junkers wiring diagram.
• Look for damage: Trace the cable between boiler and controller for cuts, crushed sections, or signs of moisture ingress; repair or replace as needed.
• Test with link: If allowed by the service manual, temporarily disconnect the controller and fit a test link or simple on/off thermostat to verify boiler response (technician only).
• Controller reset: Reset the room controller to factory settings and reconfigure according to Junkers instructions.