The equipment. Age, model, refrigerant type, condition of the coil and blower, electrical components, refrigerant charge, and whether it's performing the way it should for its age.

Airflow. Temperature split across the coil, static pressure, and whether the system is actually moving the air it's designed to move. Restricted airflow is behind a surprising share of comfort complaints and it's frequently missed.

Humidity and run time. Whether the system is running long enough to dehumidify. Equipment that's oversized for the house cools to temperature quickly, shuts off before it's pulled the moisture out, and leaves you cool and damp. No thermostat setting fixes that.

The duct system. Leaks on the return side pulling unfiltered attic air into the house, disconnected or collapsed runs, crushed flex, insulation condition, and whether rooms have adequate return air.

Room-by-room behavior. Where the temperature differences actually are, and whether the cause is duct, envelope, or equipment.

Sizing. If replacement is on the table, whether the existing capacity is right for the house as it stands now. Many homes have systems sized for the house as originally built, before window replacements, insulation upgrades, or an addition. Replacing like-for-like isn't automatically correct.

Thermostat and controls. Placement, settings, and whether what's installed is capable of managing humidity.