The display shows A9; water temperature fluctuates, or the unit shuts down intermittently.

Safety: Turn off power at the breaker and close the gas supply. Confirm the water is not scalding before touching any piping.

• Locate DHW outlet sensor: Remove the front cover. Find the sensor on the domestic hot water (DHW) outlet pipe, usually clipped to the pipe or inserted into a sensor well.
• Check mounting: Ensure the sensor is firmly attached to the pipe and insulated as designed. A loose sensor can cause erratic readings.
• Inspect wiring: Look for kinks, cuts, or burn marks on the cable. Verify the connector is fully seated on the control board.
• Test sensor: Disconnect the sensor and measure its resistance with a multimeter. Compare to Buderus NTC charts. Replace the sensor if readings are out of specification.
• Replace if needed: Install a new Buderus DHW outlet sensor, ensuring correct orientation and secure attachment to the pipe.
• Reassemble and test: Re-fit the cover, restore gas and power, and run hot water. Confirm stable temperature and that the A9 code does not reappear.