Room (Suction Air) Thermistor Abnormality

What it means

This code appears on Daikin split, ducted and cassette air conditioners and means the thermistor that measures the room air entering the unit is giving an implausible reading because of an open or short circuit. Without it the unit cannot tell when the room has reached the set temperature, so it usually stops. The same code can mean something else on other brands.

Models covered: Daikin split air conditioners and Daikin ducted/cassette (SkyAir) units, which share the same error code system; the code shows on the wired remote or via the wireless remote check function.

Related codes: C4A1H9

Symptoms3

• Code C9 appears and cooling stops.
• Before the code, the unit may not cycle off even when the room is very cold, or cuts out too early.
• Irregular compressor and fan behavior.

Possible causes4

• A loose or corroded sensor connector on the board.
• A failed thermistor from moisture or age.
• A cut or pinched sensor wire after installation or cleaning.
• A faulty sensor input circuit on the indoor PCB.

How to diagnose5

1. Switch the unit off at the breaker for a few minutes, then restart.
2. If the unit was recently cleaned or opened, the sensor connector may not have been refitted properly.
3. A technician inspects the sensor, its wire and its connector on the board.
4. The technician measures the sensor resistance at room temperature and compares it with the Daikin table.
5. If the sensor is good and the code persists, the board is checked.

How to fix5

1. Reseat and clean the sensor connector.
2. Repair the damaged wire.
3. Replace the room air thermistor with the correct part for the model.
4. Replace the indoor PCB if its sensor input is proven faulty.
5. Run the unit and confirm it cycles correctly at the set temperature.

Repair cost

In Saudi riyals; the highlighted part is the expected range for this fault.

The sensor is cheap and easy to fit on most models; the cost only rises if the board needs replacing.

Prices are rough estimates including the part and the technician’s fee, and vary with brand, model and part source.
Prices last updated: September 2026

Frequently asked questions