Completed seasonal maintenance on a Carrier central AC system for a customer in Brownfield today. Unit was a 4-ton, 14 SEER model installed in 2018. Cleaned condenser coils which had accumulated significant dust from recent wind storms. Checked refrigerant levels - slightly undercharged, added 8oz of R410A to bring to manufacturer specifications. Inspected and cleaned blower wheel and evaporator coil. Replaced air filter with customer-supplied MERV 10 filter. Tested all electrical connections and tightened several loose terminals. Calibrated thermostat which was reading 3 degrees higher than actual temperature. Applied rust inhibitor to cabinet exterior where paint was beginning to deteriorate. System is now operating at peak efficiency with 19°F temperature differential between supply and return. Recommended scheduling heating maintenance before winter season. Customer was very satisfied with thorough service and system performance improvement.