Car AC cold on one side, warm on the other

Car AC cold on one side, warm on the other — how to diagnose and fix

If your car has dual-zone heating cooling and you have AC cold on one side and warm on the other, the cause is most often a bad blend door actuator. But there are also other reasons depending on how the system was designed

How dual zone heating/cooling systems work

Most cars have a single heater core and a single AC evaporator core

Evaporator core

(although there are exceptions. See below). Once the air is heated or cooled, the air flows through two “blend doors.” The blend doors mix the hot or cold air with interior or outside air to provide the desired temperature. If you want cold on both sides, both blend door actuators will prevent the chilled air from the AC evaporator from mixing with hot air coming off the heater core.

The heat setting slider or dial connects to the HVAC control head and the control head directs the blend door actuators to open fully, partially or close fully. So, when you wind up with a car AC cold on one side, warm on the other, it’s usually a bum blend door actuator.

Diagnose the blend door actuators

A blend door actuator is an electric motor that’s 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. Then observe the motor as you change the heat setting. You should see some movement. 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.

Replace the blend door actuator

Some HVAC systems automatically recalibrate after replacing a blend door. But others require a scan tool recalibration. Many independent shops can do the calibration for you; you don’t have to go to the dealer.

Some vehicles have two AC evaporators

Some vehicles have two evaporator cores; one for each side. In those applications, there are also 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. If the evaporator isn’t cold, then you most likely have an expansion valve issue.

In some cases, it can be a low refrigerant charge

If a few rare instances, some carmaker will cool one side of the vehicle from a portion of the evaporator core and the other side from the end of the evaporator core. In rare instances where the system is really low on refrigerant, the middle of the evaporator may produce enough cool for one side of the vehicle, but the end of the evaporator picks up too much heat and can’t cool the other side of the vehicle.

©, 2021 Rick Miuscoplat