Furnace shows a flue temperature sensor error; unit may shut down shortly after ignition.

Safety first: Turn off power and gas. Allow flue components to cool fully.

• Inspect flue sensor: Locate the flue gas NTC sensor clipped to or inserted into the flue outlet near the fan. Ensure it is firmly attached and not hanging loose.
• Check wiring: Look for cuts, burns, or loose connectors on the sensor cable. Reseat the plug on the control board.
• Test sensor resistance: With power off, disconnect the sensor and measure resistance. Compare to Sime specifications. Replace if out of range.
• Check for flue blockage: If the sensor is repeatedly overheating, the flue may be partially blocked. Have a qualified technician inspect and clean the flue system.