Driver's side has no cold air

A month ago my
car's air conditioner started to blow chilled air on passenger side and at the rear vents only and the
drivers side center and left vent has regular air blowing. Problem is the same in dual temperature control mode and the in recirculate mode. Most of the time when the car is turned on the rear defroster led flashes for about a minute and then goes off. All of the air conditioner functions are disabled during that time.
I can turn on the air conditioner (auto or manual mode) and I get chilled air at passenger side and rear vents.

How do I troubleshoot this? An auto parts store checked and told me my pressure was 37 psi and that was okay.