Daikin furnace shows C9; unit may operate erratically or shut down on temperature sensing error.

Safety: Disconnect power before accessing the coil compartment or control board.

Steps to check:

• Locate coil/discharge sensor: The sensor is typically clipped to the evaporator coil tubing or mounted in the supply air plenum above the furnace.
• Check attachment: Ensure the thermistor is firmly clipped to the coil or inserted fully into the plenum so it can sense temperature correctly.
• Inspect wiring: Follow the sensor leads back to the Daikin PCB. Confirm the connector is secure and wires are not damaged.
• Resistance test: With power off, disconnect the sensor and measure resistance. Compare to Daikin’s chart. Replace the sensor if readings are out of range.

After replacing or reseating the sensor, restore power and clear the C9 code. If it returns, the control board’s sensor circuit may be defective and require professional replacement.