Car AC Blows Cold on One Side and Hot on the Other

How to Fix Car AC Blowing Cold on One Side and Hot on the Other

When a car’s AC system blows cold air on one side and hot air on the other, it usually indicates an issue with the blend door system. Most vehicles with dual temperature controls use two blend doors to vary the ait temperature coming out of each side. The problem can be due to a sticking blend door (also called air mix) actuator, a broken or binding blend/air mix door, a broken linkage to the blend/air mix door, or a faulty HVAC control head or wiring harness.

A faulty blend door actuator is the most common cause of your AC blowing cold on one side and hot on the other

Car makers used to rely on cables to change the position of the blend door to regulate the amount of heat. Cables would bind and stick, causing all kinds of problems. So, manufacturers switched to small electric motors called actuators to vary the blend/air mix door positions. Drivers change air temperature far more often than they change the mode (defrost, floor heat, dash vents, etc.), so the temperature actuators generally fail first. When that happens, the one blend door is stuck in one position, so you get a blows cold air on one side and hot on the other condition.

This image shows a blend door mounted to the side of the heater box

Blend and mode door actuators have a feedback position sensor inside to report back to the HVAC controller so it knows that the door has actually moved to the proper position. If the actuator fails, it usually fails in full-on or full-off.

So your first step in diagnosing the condition is to operate each temperature control and watch the blend door actuators to see one moves as you change the temp. The actuator that changes temperature is good, the actuator that doesn’t is bad. This test also proves out the HVAC controller. If the controller is bad, neither actuator would change positions.

Fix actuator causing blows cold one side, hot on other side condition

The blend door actuators are on the heater box behind the dash. You’ll need a shop manual to locate the correct actuator. In most cases, you’ll have to remove the glove box to gain access to the actuator.

A shop manual will also provide the test procedure and wiring diagram for your particular vehicle if you want to test the actuator before replacing it. Replacing an actuator is fairly simple. Just disconnect the electrical connector, remove the retaining screws, and slide off the actuator. Then, align the shaft of the new actuator to the blend door axle and install it. However, before actually installing the new actuator, try rotating the blend door to make sure it’s moving properly and not binding.

Perform a blend door calibration

Refer to the shop manual to learn the actuator calibration procedure. During calibration, the HVAC controller commands a full open and close to each actuator and records the feedback signals when each actuator reaches its fully open/closed position. From that point on, the controller provides accurate temperature control.

©, 2018 Rick Muscoplat