Daikin C4 Error Code — Heat Exchanger Coil Sensor: Causes & Fix

What this code means

Daikin error code C4 indicates an indoor heat exchanger coil sensor fault. The thermistor clipped to the indoor coil is reading out of range, open, or shorted. Daikin uses this sensor to monitor coil temperature for freeze protection, defrost termination logic, and capacity control.

Common Causes

• Failed indoor coil thermistor — The NTC thermistor has drifted out of spec from age or moisture exposure.
• Sensor slipped out of the coil fins — If the sensor clip comes loose, the board sees inaccurate temperature and triggers C4.
• Loose indoor PCB connector — Vibration loosens the 2-pin thermistor connector over time.
• Indoor PCB input fault — Rare, but possible after moisture damage.

Step-by-Step Fix

1. Turn off power to the indoor unit.
2. Open the front panel and remove the filters.
3. Remove the screws holding the right-side electrical cover.
4. Locate the coil thermistor clipped into the evaporator coil.
5. Verify the sensor is fully seated in the coil fins.
6. Unplug the thermistor from the PCB and measure resistance. At room temperature, most Daikin thermistors read about 5–10 kΩ.
7. Replace the thermistor if it reads OL, 0 Ω, or far outside the expected range.
8. Re-seat the connector, restore power, and verify C4 clears.

Parts Often Needed

When to Call a Pro

If the sensor tests correctly but C4 returns, the indoor control board likely has a bad thermistor input circuit. A technician can confirm before you order a PCB.