Carried out annual maintenance on a Lennox XC21-036 air conditioner to prevent problems. Fins on condenser coils were cleaned with a fin comb tool and Nu-Calgon CalClean coil cleanser. installed new Emerson EX valves in lieu of the corroded service valves. 8 ounces of R-410A refrigerant was injected to produce 10°F subcooling after the refrigerant charge was checked by measuring the superheat and subcooling readings. All electrical parts, such as run capacitors, contactors, and safety controllers, were tested. A correct condensate drainage from the evaporator coil was confirmed. Blower and condenser fan motors were lubricated with Supco SPL synthetic oil. New 20x20x1 MERV 8 pleated air filters were installed.