Boiler shows A9 and stops; may occur during operation or at startup

Safety: Turn off power and close the gas valve before opening the boiler casing.

• 1. Confirm DHW demand: Open a hot water tap and verify that the boiler attempts to start. If A9 appears, the DHW sensor circuit is suspect.
• 2. Locate DHW NTC: On Buderus combi boilers, the DHW sensor is usually mounted on the plate heat exchanger or DHW outlet pipe.
• 3. Inspect sensor and wiring: Check for scale buildup, leaks, or corrosion around the sensor. Ensure the connector is fully seated and free of moisture.
• 4. Resistance test: With power off, disconnect the DHW NTC and measure its resistance. Compare to Buderus NTC chart. Replace if out of tolerance.
• 5. Check for water leaks: Any leak at the plate heat exchanger or DHW sensor pocket can damage the sensor and wiring. Repair leaks and dry the area thoroughly before installing a new sensor.
• 6. Control board diagnosis: If a new sensor does not clear the fault, the control board may not be reading the sensor correctly. This requires professional diagnosis and likely replacement of the Buderus control module.

Note: Working on DHW components may involve draining part of the system; ensure isolation valves are closed and pressure is relieved before removing sensors.