Heater shows A81; unit may not respond to thermostat or control commands.

Safety: Turn off power before working on control wiring.

• Check room controller: Verify that the Buderus room thermostat or controller display is powered and not showing its own error codes.
• Inspect EMS bus wiring: At the heater, check the low-voltage bus terminals. Ensure wires are firmly connected and not shorted. Follow the cable to the controller and inspect for damage.
• Polarity and connections: Confirm that the bus wires are connected with correct polarity as per Buderus wiring diagrams.
• Test bus continuity: With power off, measure continuity of each bus conductor from heater to controller. Repair or replace damaged cable.
• Reset controller: Power-cycle the controller and heater. In some models, a factory reset of the controller may be required (follow the controller manual).
• Replace controller if needed: If wiring is good and the controller is unresponsive or repeatedly fails, replace with a compatible Buderus room controller.