Daikin furnace shows C4; unit may not start or may shut down unexpectedly.

Safety: Turn off power to the furnace before accessing sensors or the control board.

Steps to check:

• Locate return air sensor: On many Daikin furnaces/air handlers, the return air thermistor is mounted near the blower inlet or in the return duct, wired back to the main PCB.
• Inspect wiring: Follow the sensor wires to the control board. Ensure the plug is fully seated and the wires are not cut, pinched, or chewed.
• Check sensor placement: Make sure the sensor is not touching a hot surface or located where it can be directly heated by the heat exchanger, which can cause false readings.
• Resistance test (qualified persons): With power off, disconnect the thermistor and measure its resistance. Compare to Daikin’s temperature/resistance chart. An open or shorted reading indicates a bad sensor.

If wiring is intact and the thermistor tests bad, replace it with the correct Daikin return air thermistor. If the sensor is good but C4 persists, the main control board may need replacement.