Gas and Oil Home Heating Furnaces - AC unit not cooling after serviced

A little over one month ago I had an AC tech come out to check the unit, etc. because the unit was making a noise. They replaced the fan, checked the freon and said it was overcharged and was damaging the compressor. The unit now runs more than it use to and now that the temps are in the 90s, it won't cool below 82 degrees. The air is only cool, and not cold. I believe that they removed too much freon, or they didn't tighten the fittings enough and it is leaking. They claim that the compressor is slowly failing and can't keep up and that I should only expect a 20 degree differential between outside and inside at most. One other service company claimed that the compressor either works or doesn't and that they won't slowly fail to perform.

Any opinions?

Call a reputable service company in your area and have the system re-evaluated. A compressor can continue to operate inefficiently even though failing. However, there are ways to test and assess compressor function. Yes, an overcharge will cause premature failure of a compressor. But how much overcharge and how long has it been there? Yes, the technician could have left the schrader valve caps loose enough to lose freon if the valve stem leaks. As a rule of thumb, a 20 degree differential between outside temperature and inside temperature is valid for residential ac but is not necessarily an indicator of how efficiently your system is working. Are the coils clean? Is your filter clean? Is there enough air flow? What numbers did the technician get when he evaluated your system? I record the following at a minimum: Outdoor temperature; indoor dry bulb and wet bulb temperature; temperature drop across the indoor coil; suction pressure; liquid pressure; degrees sub-cooling and superheat; compressor LRA and RLA; and I do a few other checks as a standard evaluation. By the time I have done this much I have more than enough information to know to investigate further if needed.