Car AC blows cold on one side and warm on the other: Causes

Learn what causes your car’s AC to blow cold on one side and warm on the other

Modern vehicles often come equipped with dual-zone climate control systems designed to provide personalized comfort for both driver and passenger. However, when the AC blows cold on one side while the other side remains stubbornly warm, it can lead to an uncomfortable ride and frustrated occupants. This article delves into the various causes of this common automotive issue, exploring the components involved and potential solutions.

Understanding Dual-Zone Climate Control

It’s important to understand how dual-zone climate control systems work so you can diagnose and fix them. In most dual-zone system, hot engine coolant is always flowing through the heater core. Carmakers want the heater core hot so you can use some of that hot air to temper the cold air coming off the evaporator.

When the driver and passenger select their desired temperature, the dual-zone system operates separate “blend door actuators” to adjust the air temperature coming out of the ducts. At the full cold position, the blend door actuator closes off all hot airflow from the heater core, allowing only cold air from the evaporator. Likewise, at the full hot setting the blend door actuator moves to close off any cold air coming off the evaporator.

Notice the separate actuators for the driver’s side and passenger side. If an actuator fails, The AC blows cold on one side and warm on the other

AC blows cold on one side and warm on the other: the most common causes

Actuators are usually flat motors secured by screws or 8mm hex bolts.

A faulty blend door actuator is the most common cause

A blend door actuator is an electric motor attached to the heater box. Some connect directly to the blend door, while others connect to the blend door via linkage.

Grab a diagram from a shop manual to locate the blend door actuators for your vehicle.

With the engine and AC running, change the temperature on the side with the warm air while watching the proper actuator. You should see the actuator shaft moving. If you don’t, the actuator is either not working or the blend door is stuck or broken. In that case, remove the actuator and try moving the blend door by hand. If the door moves by hand, but the actuator doesn’t move, it’s most often caused by a faulty actuator. Purchase a replacement and install it.

Replace the blend door actuator

Some HVAC systems automatically self-frecalibrate after replacing a blend door. But others require a scan tool recalibration. Many independent shops can calibrate the system for you; you don’t have to go to the dealer.

However, Some vehicles have two AC evaporators

Some vehicles have two evaporator cores, one for each side. Those applications also have two expansion valves, so diagnosing the system gets a bit more complicated. Consult a shop manual to see if your vehicle has multiple AC evaporator cores. If so, locate the AC lines going to the evaporator on the warm side of the vehicle. If the evaporator lines are cold, then the problem is in the blend door actuator or blend door. You likely have an expansion valve issue if the evaporator isn’t cold.

In some cases, it can be just a low refrigerant charge

In a few rare instances, some carmakers will cool one side of the vehicle from the upper portion of the evaporator core and the other from the lower portion. If the system is low on refrigerant charge, one portion of the evaporator can provide cooling while the other doesn’t. In those cases, the fix is to find and fix the leak and then recharge the system.

©, 2021 Rick Miuscoplat