Boiler reports flue gas sensor or flue temperature fault; may shut down to prevent overheating.

Safety: Turn off power and gas. Flue system work must comply with gas safety regulations; if in doubt, call a professional.

Steps:

• Locate flue sensor: On condensing Bosch boilers, the flue gas NTC is usually clipped into the flue hood or outlet of the primary heat exchanger.
• Check seating: Ensure the sensor is properly inserted into its pocket or clip and making good contact.
• Inspect wiring: Look for heat damage or brittle insulation near the flue. Reseat the connector at the PCB.
• Measure resistance: With power off, disconnect and test the sensor resistance at room temperature. Replace if out of spec.
• Check for blockage (visual only): From outside, visually inspect the flue terminal for obstructions (ice, leaves, nests). Do not dismantle the flue unless qualified.

Warning: Persistent flue temperature faults may indicate serious combustion or flue issues. A Gas Safe / certified engineer must check combustion, flue integrity, and heat exchanger condition.