Daikin Air Conditioner Error Code C9

The C9 code indicates a suction air thermistor abnormality on the indoor unit. The thermistor is a temperature sensor that monitors return air entering the evaporator, and the PCB has detected that the sensor circuit is either open (disconnected or broken) or shorted. The system halts operation because it cannot obtain a valid temperature reading. Diagnosis begins with checking the connector at the PCB and measuring thermistor resistance.

The C9 fault will typically prevent the unit from running in normal cooling or heating mode, which is intentional. Operating without a valid suction air temperature reading could cause the system to behave erratically or damage refrigerant-side components. You should not attempt to bypass the fault, and the unit should remain off until the thermistor issue is resolved.

A power reset may temporarily clear the C9 code, but if the underlying thermistor fault still exists the error will return as soon as the unit attempts to operate and reads the sensor again. Resetting the unit without repairing the cause is not a lasting solution. The thermistor connector and sensor resistance should be checked before assuming the reset has resolved the problem.

A homeowner comfortable with basic electrical work can safely check whether the thermistor connector is fully seated on the indoor PCB, which sometimes clears the fault. Measuring thermistor resistance requires a multimeter and knowledge of how to interpret NTC resistance values. Replacing the thermistor itself is a straightforward repair, but accessing the indoor unit PCB and wiring should only be attempted if you are confident working around low-voltage electronics. If the fault points to the PCB, professional service is recommended.

If the fix is simply reseating a loose connector, there is no parts cost and only a minimal service call fee, typically in the range of $75 to $150 depending on your location. Replacing the suction air thermistor itself is an inexpensive part, and the total repair cost including labor usually falls between $100 and $250. If the indoor PCB requires replacement, costs can rise to the $300 to $600 range or higher depending on the unit model.