AC Running But Not Cooling: Troubleshooting Guide
The steps below follow the same diagnostic sequence that HVAC technicians use, starting with the most common and easily fixable causes and working toward problems that require professional equipment and expertise.
Step 1: Check Your Thermostat Settings
Verify the thermostat is set to "cool" (not "heat" or "fan only") and the target temperature is at least 3 degrees below the current room temperature. On programmable and smart thermostats, check whether an override, schedule conflict, or geofencing setting is interfering with normal operation. If the display is blank or flickering, the thermostat may have lost power due to dead batteries or a tripped float switch on the condensate drain. Replace the batteries and reset the thermostat to see if cooling resumes.
Also check the fan setting. When set to "on" instead of "auto," the blower runs continuously even when the compressor cycles off, which pushes unconditioned air through the vents during off cycles and makes it feel like the system is not cooling. Switch to "auto" so the fan only runs when the system is actively cooling. On smart thermostats, also check for software updates that may have changed the configuration or any away-mode settings that are limiting system operation.
Step 2: Inspect and Replace the Air Filter
A severely clogged air filter is the single most common cause of poor cooling performance. When the filter is blocked, airflow across the evaporator coil drops below the level needed for efficient heat exchange. The coil gets too cold, may freeze over, and the system blows weak, barely cool air. Even a moderately dirty filter can reduce cooling output by 5 to 15 percent, and a severely clogged filter can reduce it by 30 percent or more.
Pull the filter out and hold it up to a light. If you cannot see light through it, replace it immediately ($5 to $30 for most standard sizes). After installing the new filter, give the system 30 to 60 minutes to recover before judging whether cooling has improved. If the coil was frozen, you may need to run the fan on "fan only" mode for one to four hours to let the ice melt before restarting cooling mode. Place towels around the base of the air handler to catch water from the melting ice.
Step 3: Check the Outdoor Unit
Go outside and verify that the outdoor unit (condenser) is operating. The fan on top should be spinning and you should feel warm air being expelled upward. If the fan is not running but you can hear a humming sound, the capacitor or fan motor may have failed. If the entire unit is silent, check your electrical panel to see if the outdoor unit breaker has tripped. Reset it once. If it trips again immediately, do not reset it a second time because this indicates a short circuit that requires professional repair.
Also check for obstructions. The condenser needs at least two feet of clearance on all sides to dissipate heat effectively. Clear away any vegetation, lawn clippings, leaves, or debris that may be blocking airflow through the fins. If the fins themselves are heavily caked with dirt, a gentle garden hose rinse (never a pressure washer) can restore airflow. Spray from the inside out if possible, working from the top down to flush debris away from the coil rather than pushing it deeper in.
Step 4: Inspect for Ice on the Evaporator Coil
Check the copper refrigerant lines where they enter the indoor air handler. If you see frost or ice on these lines, the evaporator coil is frozen. This prevents heat exchange and results in warm air from the vents. Turn the system off completely (or switch to "fan only") and let the ice melt, which may take one to four hours depending on how much ice has accumulated. A severely frozen system where ice has traveled down the refrigerant lines to the outdoor unit may take up to eight hours to thaw completely.
A frozen coil is almost always a symptom of another problem: a dirty filter (which you already checked in Step 2), low refrigerant from a leak, a failing blower motor not pushing enough air, or a stuck expansion valve. If the coil refreezes after thawing, you need a technician to find the root cause. See our full guide on why your AC freezes up for the complete list of causes and costs.
Step 5: Listen for Unusual Sounds
With the system running, listen carefully at both the indoor and outdoor units for sounds that indicate specific failures. A hissing or bubbling sound near the indoor unit often indicates a refrigerant leak. Clicking or buzzing from the outdoor unit suggests a failing contactor or capacitor. Grinding or screeching indicates a failing motor bearing. A loud rattling or banging from the outdoor unit can mean a failing compressor or loose hardware.
These sounds help you communicate the problem to a technician and can help you evaluate their diagnosis. See our guide on AC noises and what they mean for a detailed sound-by-sound reference with urgency levels and repair costs for each type of noise.
Step 6: Call a Licensed HVAC Technician
If the steps above do not resolve the problem, the issue requires professional equipment to diagnose. A technician will check refrigerant levels with manifold gauges, test electrical components with a multimeter, measure airflow across the evaporator coil, and inspect the compressor for internal failure. A standard diagnostic service call costs $75 to $150, with many companies crediting this fee toward the repair if you proceed with their recommended fix.
When calling for service, mention what you already checked and any unusual sounds or symptoms you noticed. This helps the technician prepare for the likely repair and may reduce diagnostic time. Get at least two quotes for any repair quoted above $500, as pricing varies significantly between companies, especially for major repairs like compressor or coil replacement. See our AC repair cost guide for current pricing on every common repair so you can evaluate quotes with confidence.
When the Problem Is Extreme Heat, Not Your AC
During extreme heat events (temperatures above 100 degrees Fahrenheit), even a perfectly functioning AC may struggle to maintain your set temperature. Most residential systems are designed to cool your home 20 to 25 degrees below the outdoor temperature. If it is 110 degrees outside, the best your system can do is around 85 to 90 degrees indoors. This is normal operation, not a malfunction. The system will run continuously but maintain a steady indoor temperature that is 20 to 25 degrees cooler than outside.
If your area regularly experiences temperatures above 100 degrees and your current system consistently fails to keep up, you may need a larger unit or a more efficient system. Homes with poor insulation, large west-facing windows, or inadequate attic ventilation lose cooling capacity faster than the system can replace it during extreme heat. Addressing these building envelope issues (adding insulation, installing reflective window film, improving attic ventilation) can be more cost-effective than upgrading the AC system. Our guide on AC replacement by tonnage explains how to size a system for your home and climate when replacement is the right answer.
Start with the free checks: thermostat settings, air filter, outdoor unit operation, and breakers. These solve roughly half of all "not cooling" complaints. If the coil is frozen, let it thaw and check for an underlying cause. Any hissing sounds, repeated breaker trips, or complete compressor silence requires a professional technician.