Error A51 appears; burner does not ignite or shuts down quickly.

Safety: Turn off power and close gas supply. If connected to a storage tank, ensure water is not excessively hot before handling the sensor well.

• Locate DHW sensor: For indirect tanks, the sensor is inserted into a dry well on the tank. For combi units, it may be on the internal DHW heat exchanger.
• Check insertion depth: Ensure the sensor is fully inserted into the well and secured with the clip or clamp provided.
• Inspect cable: Look for cuts, abrasions, or signs of overheating. Verify the connector at the control board is tight.
• Measure resistance: Disconnect the sensor and measure resistance. Compare to Buderus NTC values at the measured water temperature. Replace if out of spec.
• Replace sensor if faulty: Install a new Buderus-approved DHW sensor, ensuring proper insertion and strain relief on the cable.
• Restore operation: Reassemble covers, restore gas and power, and verify that the A51 code clears and DHW temperature control is stable.