Well first you're going to need a set of gauges, not the single gauge that comes on the easy fill cans, the actual set of manifold gauges with 2 pressure gauges for Hi/Low side and shutoff valves and a fill line. That will allow you to hook up and actually read the Hi/Low side pressures and be sure that it is just low on freon instead of potentially something else. Also not sure what year they started making the switch from R-12 to the more "environmentally friendly" (read as doesn't work as well) refigerants, but if yours is R-12 it could cause some problems as you can't get that stuff anymore with an HVAC license and its about 100 bucks a pound last time I heard. They do make some newer stuff that works for multiple system types, and also some conversion kits to convert an R-12 system to 134a or similar, however those kits require cracking open the system, at which point you also should replace out the dryer, as well as requires a vac out in order to recharge.
In a rather rambling way what I'm getting at is that if you really want to be sure that you're going down the right path for fixing you need a set of manifold gauges to be sure. Now it may be that its just low, and a can of freon will do it, however at this point its hard to tell. Whichever way you go, when you go to charge the system you will need to reset that code so the AC compressor can cycle on, you can't charge without the compressor cycling on and off, and if you do go ahead and get the ready to charge kits at the big box retailer or auto parts store, I would avoid the stuff with "leak stop" or similar names....I don't know the details but all HVAC techs and mechanics I know who work on AC a lot say to stay the hell away from it, all its going to do is cause seal failures in the future and cause longer term, large dollar repairs. If you drop a can in and it fixes the problem temporarily but the error comes up again in a week or two, buy a can with a leak tracer(UV dye) to help pinpoint the leak in the system.
