Why Is My AC Not Cooling? A Step-by-Step Guide

Step 1: Confirm What the Thermostat Is Doing

Set the mode to COOL, the fan to AUTO, and the setpoint at least three degrees below the current room temperature. Then wait a few minutes and listen. Does the indoor blower start? Does the outdoor unit start? Those two answers split the problem in half.

If nothing starts at all, the issue is more likely electrical, control-related or a tripped safety. If the indoor blower runs but the outdoor unit does not, the problem is probably at the condenser. If both run and the air still is not cold, the refrigerant circuit, airflow or capacity is in question.

Step 2: Feel the Air and Measure the Difference

Hold your hand at a supply register and then at the return grille. On a properly operating system, supply air is noticeably colder than return air — commonly in the range of 15 to 20 degrees of difference, though this varies with humidity and conditions.

If there is almost no difference, the system is not removing heat, which points toward the refrigerant circuit or a compressor that is not running. If the difference is large but the house is not cooling, the issue is more likely airflow volume, duct loss or capacity relative to load.

Step 3: Check the Filter and the Returns

Pull the filter and hold it up to a light. If light barely passes through, replace it. Then walk the house and confirm return grilles are not blocked by furniture and supply registers are open. This step resolves more cooling complaints than any other homeowner action.

Step 4: Look for Ice

Check the insulated copper line at the outdoor unit and, if accessible, the indoor coil. Any frost or ice means the system needs to be shut off and thawed before it can be diagnosed accurately — and it means an airflow or refrigerant problem exists underneath.

Step 5: Inspect the Outdoor Unit

Do not open the electrical panel on the unit. Observation from outside the cabinet is enough to give a technician useful information.

• Is the fan spinning when the thermostat calls for cooling?
• Is the unit humming without the fan turning?
• Are the coil fins visibly packed with dust, grass or cottonwood?
• Is there vegetation, fencing or stored material within two feet?
• Has the disconnect near the unit been pulled?
• Are there scorch marks or a burning smell?

Step 6: Check the Breakers and Safeties

Look at the electrical panel for a tripped breaker serving the indoor or outdoor equipment. Reset it once. If it trips again, stop — a repeatedly tripping breaker indicates a fault that needs diagnosis, not another reset.

Also consider the condensate float switch. If the drain has backed up, the switch intentionally shuts the system down and often leaves the thermostat looking dead. Standing water in the pan is the giveaway.

Step 7: Consider the Load

On an extreme afternoon, even a healthy system may hold the house a set number of degrees below outdoor temperature rather than reaching a low setpoint. Ask yourself whether the system used to keep up under similar conditions. If performance has changed, something has changed in the equipment or the duct system.

What to Tell the Service Provider

Summarize your findings: what starts and what does not, whether air at the vents is cold, whether there is ice, whether the outdoor fan spins, whether a breaker has tripped, and the approximate age of the system. That summary shortens diagnosis and helps route your request accurately. Have your 5-digit ZIP code ready when you call.