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If you have a manual style A/C and the vents move to defrost ecah time you go to idel, yes it is the blend door system. If you stay on the air the whole time, then no. The issue will eitehr be the refrigerant is low, the expansion valve/orifice tube is bad, or the engine fan is not engaged and allowing the condenser to cool down. Most likely, there is a fault with the refrigerant and the system has leaked out just a bit too much from a dried out o-ring.
I'm in Florida and see that a lot from the constant A/C use in states like ours. The A/C stays on all the time and the o-rings dry out and will seep the gas out over the span of a few years to where it just gets a little too low.
Best thing for the ultimate cooling is to have the refrigerant recovered, the system vacuumed down, then recharged to spec. The reason is as the o-ring leaks, it will suck in air as well and won;t blow as cold when you charge it back up. You want oit to cool the cab fast and the best way to do that is with no air in the system so getting it recovered and recharged instead of just adding a can will do the A/C a lot better and the people in the vehicle as well.
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