What Causes the Car AC to Only Blow Air at the 4th Speed?

[DeJuliet]

The reason why the car AC only blows air at the 4th speed is due to a damaged blower motor resistor. The car AC may also experience the following issues: 1. Reduced airflow: A dirty cabin air filter can cause the blower motor to make noise, but the airflow remains weak. 2. Inability to adjust airflow direction: Damaged air deflectors or airflow direction control switches can prevent airflow direction adjustment. 3. Abnormal airflow: A frozen evaporator can cause sudden loss of airflow at high speeds, though the blower motor can still be heard running. Restarting the AC may restore airflow. 4. Uneven airflow between left and right vents: It is normal for the right vent to have stronger airflow than the left vent.

[StGrace]

I've been driving for over twenty years and have experienced several instances where the air conditioning only blows air at the highest setting, usually during hot summers or humid weather. At those times, the lower settings don’t produce any airflow at all, and only the 4th setting delivers noticeable air. This issue typically stems from a faulty blower motor resistor, as the resistor is responsible for regulating the fan speed—lower settings rely on the resistor to reduce speed, while the highest setting directly connects to the current. When the resistor block ages or burns out, the lower settings stop working. Once, I also encountered a poor switch contact, where a light tap temporarily restored function, but it didn’t last. I recommend first checking if the fuse is intact, then focusing on the resistor module. DIY repairs aren’t difficult, but safety comes first—especially since losing AC control at high speeds can be dangerous. I always get it fixed promptly to avoid overheating in the car, which can affect visibility and comfort. Spending a little money to replace the part solves the issue.

[LeLondyn]

Last time I repaired my car myself, I noticed the AC strangely only blew air at speed 4, with no response at other settings. After researching forums and short videos, I concluded it was a faulty blower motor resistor - these often fail due to dust or heat wear, blocking lower speed circuits. When I removed the dashboard panel, I indeed found burn marks. I ordered a matching resistor online and replaced it for about 80 yuan. The DIY process was quite interesting, but you must be careful not to touch sensitive circuits. Insufficient airflow during commutes makes the cabin stuffy and affects concentration. Fortunately, this is easy to diagnose - always start with the simplest components first. If the resistor checks out fine, then inspect the fan switch or wiring connections next.

[MacAyla]

As a homemaker, I shuttle my kids daily, and air conditioning issues immediately raise safety concerns for me. When it only blows air at the 4th speed setting, my children complain about overheating in the back seat, likely because the lack of lower fan speeds makes it difficult to reduce the temperature, causing discomfort. The root cause is usually a faulty blower motor resistor, which disrupts the slower speed settings. I went straight to the auto parts factory to replace the component, which took half an hour. Overlooking such details during routine maintenance poses significant risks, especially during long trips when stable climate control is essential. I remind fellow parents not to ignore minor malfunctions and to prioritize professional inspections.