A/C blows cold on one side and warm on the other: 2 common reasons

By Vlad Samarin May 22, 2023
If the air from the vents in your car is cold on one side and warm on the other side when the A/C is ON, it is commonly caused by one of the following reasons:
  1. A malfunctioning blend door actuator or a mechanical problem with the blend door. There are several doors in the HVAC system of a car that control hot/cold airflow distribution through air ducts. The doors are controlled by devices called blend door actuators. If there is a mechanical problem with one of the blend doors or its actuator is malfunctioning, it could cause the air to be warm on one side and cold on the other side when the A/C is ON.

  2. Low refrigerant level in the air conditioning system. The A/C system of a car is filled with refrigerant (R134 or R1234yf), commonly known as Freon. If some amount of refrigerant leaked out, it could cause inadequate cooling, often on one side. In this case, air from the vent on one side often feels warmer but not hot. The vent on the other side could blow cold air, but it also doesn't feel freezing cold like in a fully functioning A/C system.
From our experience, these two reasons are the most common. Other possible causes include incorrect settings in the Dual-Zone air conditioning system, problems with HVAC air ducts, and issues with the HVAC control module or wiring.
What is the solution? The solution is to take your car to your dealer or a qualified mechanic to have the A/C system properly diagnosed.
Technician checking the A/C system temperature Technician is checking the A/C system temperature.
If you want to know more, you can do some research on YouTube and forums dedicated to Make and Model of your car.

For example, we know that a faulty blend door actuator was very common in some Ford vehicles, and you can find plenty of YouTube videos describing the symptoms and repair steps.

How much will the repair cost? When you take your car to a dealer or a repair shop, they will typically charge you a flat A/C diagnostic fee and then present you with an estimate.

If the A/C system is low on refrigerant and your mechanic is able to find the leak, the repair price will depend on which part is leaking. For example, if the A/C condenser is leaking (which is common), in an average car, it will cost 1.5-2 hours of labor to replace plus the cost of part ($80-$350) plus refrigerant.

If your mechanic won't be able to find the leak in the A/C system, they will recharge it with fresh refrigerant and add dye into the system that will help them find the leak. Dye is a special chemical that will glow under a blue light source. If the leak is not yet showing up after recharging the system, your mechanic may advise you to come back later for further diagnosis.

If it's a blend door actuator that is faulty, the part is not very expensive (often, less than $100). The labor cost will depend on the difficulty. In some cars, they could charge 0.7-1.5 hours to replace the blend door actuator. In other cars it could be more.

What is a blend door actuator? There are several air doors that direct the flow of air through the system in the car's heating ventilation and air conditioning (HVAC) system.
HVAC system of a car diagram Air flow in the HVAC system of a car.
In many cars these doors are operated by electrical actuators.

For example, the actuator in the photo below controls the Fresh Air/Recirculation mode door. Electric actuators are typically called door actuators or blend door actuators.

Problems with blend door actuators are common in many cars.
Blend door actuator Blend door actuator.
As you know, a bad blend door actuator can cause the A/C to blow cold air only on one side of the car. Another common problem with blend door actuators is when the gears inside wear out and start skipping.

The typical symptom of this problem is a repeated clicking noise coming from under the dash. Often the noise lasts for a few seconds when changing the HVAC modes or turning on the heat or A/C.

Blend door actuators replacement: The main difficulty with replacing one of the blend door actuators is their location under the dash. In some cars, the actuators can be accessed through the glove box or from the driver's side. In some trucks, the actuators are placed at the bottom of the HVAC unit. In some cases a dashboard might even need to be removed to get access, although there could be ways around it. If you prefer a DIY repair, YouTube has many videos on blend door actuators symptoms and replacement.