• Thermostat is in the wrong mode, set above space temperature, or on an unoccupied schedule
• Dirty or collapsed air filter starving airflow
• Plugged condensate drain and a tripped float switch (safety shut the unit down on purpose)
• Tripped breaker, blown control fuse, or a disconnect left open after other work
• Closed dampers, blocked return, or closed supply diffusers in the affected area
• Frozen evaporator coil from low airflow
• Dirty condenser coil or blocked outdoor airflow raising head pressure
• Contactor, capacitor, or control-board failure (qualified technician)
• Low refrigerant charge or a leak (licensed technician only)
• Compressor or reversing-valve failure (licensed technician only)

Bring these

• Flashlight or headlamp
• Correct-size replacement filters
• Pocket thermometer or infrared thermometer
• Wet/dry vacuum for accessible condensate drains
• Nitrile gloves, safety glasses, and hearing protection on a roof

Before touching anything

• Immediate safety check: look for smoke, burning smell, sparking, standing water near electrical equipment, or an audible refrigerant leak. Any of those means stop and call qualified help.
• Confirm you are authorized to be on the roof or in the mechanical room, and follow your site's lockout/tagout procedure
• Ask what changed: recent power loss, filter change, tenant work, or a thermostat adjustment
• Note the space temperature, setpoint, and time the complaint started

1. 1Verify the complaint at the thermostatRead the space temperature and setpoint. Confirm mode is Cool, the fan setting is correct, and the schedule is not in unoccupied setback. A blank display points to power or a tripped safety upstream.
2. 2Confirm airflow at the diffusersFeel for air movement in the affected space. No airflow anywhere is a fan or power problem; airflow in one area only is a damper, balancing, or blocked-return problem.
3. 3Inspect the filterPull the filter and hold it to the light. If it is loaded, replace it with the same size and rating, note the date, and reassess after 20 minutes of runtime.
4. 4Check the condensate drain and float switchLook for water in the pan or a wet trap. Clear an accessible drain with a wet/dry vacuum. If the float switch tripped, the unit shut itself down for a reason: clear the cause, never bypass the switch.
5. 5Look at the evaporator coil for iceIce means restricted airflow or a refrigerant issue. Switch to fan-only to thaw, correct filters and airflow, and if ice returns, escalate for refrigerant diagnosis.
6. 6Check the condenser and outdoor airflowClear leaves, debris, and stored material from around the unit. Note any heavy coil fouling for a cleaning work order. Do not remove electrical panels.
7. 7Confirm power state and documentNote breaker and disconnect positions without opening panels. Record model, serial, error codes on the display, and what you already checked, then escalate if the space still is not cooling.

• Replace a loaded filter with the correct size and rating
• Clear an accessible condensate drain and pan, then flush
• Reset a float switch after the water cause is corrected
• Reopen supply diffusers and clear blocked returns
• Remove debris from the outdoor unit and clear the service clearance
• Correct thermostat schedule, mode, and setpoint

• Bypassing a float switch to get the unit running again
• Resetting a breaker repeatedly instead of finding the fault
• Installing the wrong filter size and leaving bypass gaps
• Assuming low refrigerant before checking airflow
• Closing the call without documenting model, serial, and readings