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The expansion valve has nothing to do with this type of problem, so I question why they suggested it. Since the AC is cold on the one side but not the other, the AC system is working. However, it sounds like what is called a blend air door actuator has failed. This actuator is what is responsible for determining the temperature of air entering the vehicle.
I attached a picture of the actuator for you to see. Most vehicles refer to it as a blend air door actuator, but on this vehicle it is referred to as an air mix actuator. See the attached picture.
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What I would suggest is to remove it and check it for operation. If it fails, here are the directions for replacing it. The remaining pictures correlate with the directions.
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2010 Mazda 3 L4-2.5L
Air Mix Actuator Removal/Installation
AIR MIX ACTUATOR REMOVAL/INSTALLATION [FULL-AUTO AIR CONDITIONER]
Driver Side
1. Disconnect the negative battery cable.
2. Disconnect the connector.
3. Remove the hook (1), remove the rod (2) from the air mix actuator link.
Pic 2
4. Remove the screw.
Pic 3
5. Remove the driver side air mix actuator.
6. Install in the reverse order of removal.
Let me know if this helps or if you have other questions.
Take care,
Joe
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Thursday, August 29th, 2019 AT 7:55 PM