E05 error and furnace stops; controller may show flue temperature fault

Safety: Allow the flue to cool before touching any sensor or metal parts.

• Locate the flue gas temperature sensor (usually a probe inserted into the flue outlet or flue pipe near the boiler).
• Inspect the sensor cable for cuts, melted insulation, or loose connectors at the sensor and control board.
• Ensure the sensor is fully inserted into its pocket or sleeve and secured with the retaining clip.
• If you have a multimeter and the Defro service data, measure the resistance of the sensor at room temperature and compare to the NTC curve. An open circuit or near‑zero resistance indicates a failed sensor.
• Reset the controller and monitor operation. If E05 reappears quickly, the sensor is likely defective.

Note: Do not operate the furnace with the flue gas sensor disconnected or bypassed; this is a critical safety device.