I have a 20 year old A/C unit in my house here in Texas. Two weeks ago, the unit stopped cooling. A/C company couldn't come out until the next day and the house got pretty miserable. When the tech arrived early the next day he measured temps in the return plenum at 78, and supply plenum at 76. Obviously air was not being cooled. His diagnosis was that the evaporator coil was broken and needed to be replaced and that we should probably replace the whole system as well given the age.
Then we went outside to the condenser and he measured the R-22 levels. The readings were 58/196 (those numbers mean nothing to me, but he said it was pretty low) and the temperature in the system was 33 degrees, instead of 40 like it should be. He hooked everything back up and gave us quotes for $7200-$8600 to get a whole new system.
However, after hooking everything back up, the A/C began cooling just like it had before. My suspicion is that the line was either frozen (given the very close to freezing temperatures) or something else was preventing freon from moving through the system, and his measuring the levels caused it to start working again.
We obviously have an R-22 system, and the cost of adding R-22 is very expensive now, but it seems the rest of the system is still working fine (I just replaced a blower motor myself 2-3 months ago). I'd hate to dump $7k in a system that might have a year or more left in it before something really goes wrong. Would adding additional R-22 cause the temperature in the freon line to increase, and potentially prevent this from happening again for the short term? Would it be better to bite the bullet and just replace everything now?
Edit: After listening to the advice here, I added 2lbs of R-22 to the system. Unfortunately, everything stopped working again 8 days later and we couldn't get it to start cooling again after 36hours of turning it off and back on. We had to go ahead and replace the whole system. My assumption is that we had two problems: the R-22 was low, and the evaporator coil had gone bad and the initial problem was that the coil just hadn't completely died when the first service call happened.