What should you check when the air handler is not working? The symptom narrows the next observation but does not identify a failed part. For the indoor air handler and blower, first separate what the thermostat or normal controls request from what the indoor and outdoor equipment actually do. Note airflow, temperature change, timing, sound, odor, visible water or ice and whether the problem affects one room or the whole home, without opening a panel or reaching into equipment.
Check only thermostat indications, accessible service switches, obvious water around the unit, filter condition, sounds, odors, and vent airflow, stopping immediately for electrical, smoke, or overflow hazards. A completely silent air handler is a different branch from a running blower with no conditioning, intermittent shutdown, condensate overflow protection, frozen-coil airflow loss, or a zoning-control issue. A technician can evaluate incoming and low-voltage power, safeties, controls, blower, coil condition, drainage, heat strips if present, and communication with the outdoor equipment. Give the dispatcher the system and thermostat models, affected modes, error codes, water or ice, breaker history, and recent maintenance, then ask for a documented diagnostic before any component replacement. Air-handler location, drainage code, heat-strip configuration, humidity, flood risk, utility controls, and access rules vary by region and building type.