My AC ususally blows about 50 degree air from the vents when I turn on the AC. Last night the vent air was around 60 degrees and was noticeably warmer. The house never cooled off after it had run for about 90 minutes. It went from 90 to 86.

I have a service call this afternoon, and I suspect the system needs to be recharged. If it turns out to be something in the outside unit (like the compressor), can they just replace that part or do they normally replace the whole unit?