Central AC System GuideWhen a central air conditioner stops cooling, the cause is often a simple airflow or thermostat problem rather than a failed compressor. The Department of Energy lists dirty filters, sensor and thermostat faults, and drainage issues among the most common problems. Refrigerant and electrical faults are different: DOE advises that a trained technician handle refrigerant leaks, so those are a stop-and-call-a-pro point.
Start with the air filter
The most common and most fixable cause of weak cooling is a clogged filter. ENERGY STAR advises inspecting, cleaning, or changing air filters once a month in a central air conditioner, because a dirty filter forces the system to work harder to move air through the home.
A clean filter also protects efficiency. The Department of Energy reports that routinely replacing the air filter can lower an air conditioner's energy consumption by 5 to 15 percent. Checking and swapping the filter is the safe first step before touching anything else.
Check airflow and ducts
If the filter is clean but rooms still feel warm, airflow is the next suspect. ENERGY STAR reports that airflow problems can reduce a system's efficiency by up to 15 percent, so blocked vents, a dirty blower, or crushed ducts all show up as poor cooling.
Leaky ducts waste cooling before it reaches the room. ENERGY STAR estimates that about 20 to 30 percent of the air moving through a typical duct system is lost to leaks, holes, and poorly connected ducts. Homeowners can confirm registers are open and unobstructed, but sealing ducts inside walls is contractor work.
Thermostat, drainage, and sensor checks
The Department of Energy lists several problems that keep a unit from cooling properly, including thermostat issues, sensor problems, and drainage problems. Confirm the thermostat is set to cool and to a temperature below the room reading, and that its batteries are good.
Drainage matters too. ENERGY STAR advises checking and inspecting the condensate drain, since a plugged drain can cause water damage and raise indoor humidity, and some units shut down on a full drain pan. A clogged condensate line is often a homeowner-clearable item, unlike the electrical and refrigerant faults covered next.
Stop here: refrigerant and electrical faults
This is the point to stop and call a licensed professional. The Department of Energy warns that if an air conditioner is low on refrigerant, a trained technician should repair any leak and properly recharge the system; simply topping off refrigerant does not fix the underlying leak and is not a homeowner task. Refrigerant is handled under federal rules and requires certified equipment.
Electrical faults are the other hard stop. DOE describes electric control failure, where the compressor and fan controls wear out, especially when a system oversized for the home cycles on and off frequently. Do not open the electrical panel or the disconnect at the outdoor unit. If cooling does not return after the filter, airflow, thermostat, and drain checks, the safe next step is a professional service call rather than further do-it-yourself work.