Sensor and controller diagnosis for abnormal cycling and sudden airflow reduction in a Kia Cerato

Sifu White explains what happened

Prices, repair times and warranty terms mentioned in the video applied at the time. Ask the workshop about current arrangements.

When the air-conditioning does not cut off or release properly, the sensor is not necessarily the cause. This Kia Cerato arrived yesterday with a complaint that airflow would suddenly become very weak. Many people might immediately assume that the evaporator temperature sensor needs replacement.

The sensor was initially one of the suspects, but the customer said it had already been replaced at another workshop. Sifu White removed it and measured its resistance in ohms; the reading appeared normal. We therefore did not immediately conclude that the sensor was faulty and tried another sensor for comparison.

After testing, a reading of about 1.5 is mentioned, although its unit is unclear. The sensor is also positioned differently. The inspection suggests that the problem is more closely related to a controller or compressor control valve, so we did not replace the compressor immediately. Sifu White has previously explained that improper cut-off or release does not necessarily mean a sensor or compressor fault.

Why did Sifu White avoid replacing the compressor immediately? It had been replaced only about two years earlier, so assuming it had failed without further testing did not seem reasonable.

Why airflow can fade during a long drive

The evaporator is the component commonly called the cooling coil. Air is cooled as it passes over it. If ice builds up on its surface, the air passage can become progressively restricted. The owner may notice strong cooling initially, then less and less air during a longer drive. That sequence is different from air continuing to blow strongly but becoming warm, and the distinction helps the technician choose relevant tests.

Temperature feedback, anti-icing control, refrigerant condition, metering and airflow can all be relevant; ice alone does not prove a sensor has failed. A problem that develops after a long run may not appear during a short idle test. Record how long it takes, the temperature and fan settings, and whether airflow returns after the system has been off for a while. Verification should cover those trigger conditions without treating unsafe driving as a diagnostic requirement.

Separate component failure from power and control faults

Pressing A/C is a request; the vehicle still has to meet its operating conditions. Controllers, sensors, relays and wiring work together to determine whether a component runs. A compressor that does not start may have an internal fault, or it may not be receiving the required power or command. Before replacement, the technician needs to establish where the failure occurs in the actual circuit rather than treating the visible symptom as the diagnosis.

A fault code helps direct testing, but it does not always identify a part to buy. Actual output, connectors, ground connections and circuit behaviour under load may all need verification. Added relays or bypass wires can also change the original control relationships. After addressing an electrical fault, the air-conditioning needs to be operated again to check the original complaint. A cleared code or newly installed part is not sufficient evidence of a successful repair by itself.

The next check is still pending

The video ends before the final repair is confirmed. The remaining checks will determine what needs to be repaired or replaced.

What to tell the workshop about your own car

You do not need to guess a part name first. A clear description of what actually happens is often more useful for reproducing the complaint than requesting a particular replacement.

• Record when airflow fades and whether stopping or switching A/C off changes it.
• Mention previous wiring changes, added relays or controller replacements.
• Include the model, year, recent repairs and what changed before or after the symptom began.