Furnace indicates outdoor temperature sensor fault (for models with integrated outdoor reset or dual-fuel).

Safety first: Turn off power before working on low-voltage sensor wiring.

Steps:

• Locate the CTC outdoor temperature sensor mounted on an exterior wall away from direct sunlight.
• Inspect the sensor and cable for physical damage or cuts.
• At the furnace control board, verify that the sensor wires are securely connected to the designated sensor terminals.
• With power OFF, disconnect the sensor and measure its resistance with a multimeter. Compare to the CTC resistance/temperature chart for your model. An open or short reading indicates a bad sensor.
• Replace the sensor with the correct CTC outdoor sensor if readings are out of range.

Note: If a new sensor still reads incorrectly, the control board sensor circuit may be defective.