Boiler locks out with A8; burner does not start or shuts down quickly after ignition attempt

Safety: Switch off the boiler and isolate electrical power before removing any covers.

• 1. Reset and observe: Reset the boiler according to the user manual. If A8 reappears immediately, proceed with sensor checks.
• 2. Locate return NTC: The return temperature sensor is typically mounted on the return pipe entering the boiler's hydraulic block or heat exchanger. Refer to the Buderus service diagram.
• 3. Visual inspection: Check the sensor body and clip for damage or signs of overheating. Ensure it is firmly attached to the pipe and that any thermal paste or contact surface is intact.
• 4. Wiring check: Follow the cable from the sensor to the control board. Look for pinched, cut, or melted insulation. Reseat the connector at both ends.
• 5. Electrical test: With power off, disconnect the sensor and measure its resistance. Compare to Buderus NTC specifications. Replace the sensor if readings are out of range.
• 6. Compare flow and return readings: If you have access to diagnostic mode, compare displayed flow and return temperatures. If one is clearly unrealistic (e.g., -40°C or 130°C at room temperature), that sensor or its circuit is faulty.
• 7. Control board: If both sensors test good but the error persists, the control board may be defective. This should be handled by a qualified technician.