Water heater shows A8 and will not start; unit may attempt ignition and then lock out.

Safety: Switch off electrical power and close the gas supply before servicing. Allow the heater to cool.

• Locate the return sensor: Open the front panel. The return temperature sensor is usually mounted on the return water line or lower part of the heat exchanger.
• Visual inspection: Check for signs of water leakage onto the sensor or harness, corrosion on the connector, or mechanical damage.
• Check wiring continuity: Disconnect the sensor and use a multimeter to verify continuity from the sensor plug back to the control board connector. Repair or replace damaged wiring.
• Test sensor resistance: Measure the NTC resistance and compare with Buderus specifications for the measured water temperature. Replace the sensor if readings are out of tolerance, open, or shorted.
• Reconnect and secure: Refit the sensor and ensure it has good thermal contact with the pipe or well. Secure the harness away from hot surfaces and sharp edges.
• Power up and test: Restore gas and power, clear the fault by cycling power, and run the heater. Monitor the display to confirm the A8 code does not return.

If the sensor and wiring are confirmed good, the main control board may require replacement by a professional.