Performed seasonal preventative maintenance on a Lennox XC21-036 air conditioning system. Cleaned condenser coil fins using Nu-Calgon CalClean coil cleaner and fin comb tool. Replaced corroded service valves with new Emerson EX valves. Checked refrigerant charge by measuring subcooling and superheat values, added 8 oz of R-410A refrigerant to achieve 10°F subcooling. Tested all electrical components including run capacitors, contactors, and safety controls. Verified proper condensate drainage from evaporator coil. Lubricated condenser fan and blower motors with Supco SPL synthetic lubricant. Installed new 20x20x1 MERV 8 pleated air filters.