The other night our AC (Lennox hs26) stopped blowing cold air. The inside unit is fine, the air simply isn't cold. I tried the stick test (manually starting the outside condenser fan) and it works.

From what I've researched, this would mean the capacitor has gone out. However, I can tell that the previous owner of our home (we moved in a month ago) had also replaced the capacitor semi-recently, as it appears new (no more than a year or two old).

Is there anything else that could be causing this? Is it a possible wiring issue? Again, the AC worked fine for weeks.

Any help is greatly appreciated.

While a capacitor should last many years, its age is not a way to determining if it is faulty. If you don't know how to test a capacitor, just replace it and see what happens. They are not that expensive. Be sure you are looking at the fan motor capacitor, not the compressor capacitor. If problem persist after replacing capacitor then fan motor is faulty. Start windings of fan motor may be faulty or bearings could be worn out or corroded.