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Q: Air conditioning works, but during longer trips it becomes weaker and weaker.

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On longer trips, the air conditioning will begin to not keep the car cool, and I'll need to turn the fan higher and higher until I'm at max A/C. Once max A/C will no longer keep the car cool, I'll turn the A/C off, roll down the windows, and after a while, the A/C will be "recharged" and will again begin to push out cool air. Any idea what the problem is? Thanks!
My car has an automatic transmission.

A: First thing to do is check the refrigerant ...

First thing to do is check the refrigerant charge level, typically via gauge (high and low) pressures, although note that gauge pressures do not perfectly coincide with what is intended to be the factory specified amount of charge (measured very precisely in ounces, indeed tenths of an ounce) of R134a.

Gauge pressures will tell you if the system is clearly low, as well as give you an overall indication of the function of compressor (e.g. the compressor is "compressing"). If charge is good (e.g. if pressures are within acceptable range and STABLE), you want to check and make sure the condenser is not blocked with debris or has lots of bent fins. You want to be sure that the condenser fan works, as well as radiator fan, which also draws air over condenser.

Intermittent cooling can sometimes be caused by excess moisture in the system. The moisture can freeze in the expansion valve blocking flow of refrigerant. If your system has not been opened though, there is no reason for moisture to have gotten in except in a circumstance where refrigerant level in your system has gotten so low that the low side has drawn a vacuum and air has leaked in. At any rate, check gauge pressures first. If you would like to have this checked, a certified professional from YourMechanic can come to your car's location to inspect the AC and let you know what needs to be done.

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