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Your AC is on. The thermostat shows cooling mode. Two hours later, the house is still 80 degrees and the system is running in a loop. This problem is different from an AC that’s completely dead. The system is doing something. The challenge is figuring out whether it’s doing enough, and what’s getting in the way. Start with the things that take two minutes and cost nothing. Then work toward the more complex causes. This guide follows that order and covers the technical details most homeowners never get from a service tech, because knowing what’s actually happening inside your system is the fastest way to figure out what you’re dealing with. Check the Thermostat Fan Setting First: On vs Auto Before anything else, check the fan setting on your thermostat. Most have two options: Auto and On. When the fan is set to On, it runs continuously regardless of whether the compressor is running. This pushes unconditioned, room-temperature air through the vents constantly. When the fan is set to Auto, air only moves when the compressor is actively cooling it. If the fan is set to On and the compressor cycles off, you’ll feel air coming from the vents that’s barely cool or completely neutral. It feels like the AC is running but not doing anything, because in the fan-only phase it isn’t. Switch it to Auto and run for 30 minutes before drawing any conclusions. This setting catches a surprising number of service calls. It costs nothing to check and takes 10 seconds. Replace the Air Filter and Do It Right Now A partially clogged filter doesn’t just reduce airflow. According to the US Department of Energy, a dirty air filter can reduce AC efficiency by 5 to 15 percent. In severe cases, a fully blocked filter starves the evaporator coil of warm return air, drops the coil temperature below 32 degrees Fahrenheit, and freezes it solid. The system keeps running. Air still moves. But it’s barely cool because the coil is encased in ice. Use a MERV 8 to MERV 11 filter. Anything rated MERV 13 or higher restricts airflow too much in most residential systems and contributes to the freezing problem you’re trying to solve. On a system running 12 to 16 hours a day in summer, a standard 1-inch filter can load up in three to four weeks. Check it monthly during peak season. One thing no one tells you: running a system with no filter at all is not the solution when you run out of filters. Without a filter, dust and debris coat the evaporator coil directly. That buildup is far harder and more expensive to clean than swapping a filter. Do the Temperature Split Test Before Calling Anyone This is the single most useful diagnostic step a homeowner can do, and almost no one does it because it’s never explained. It takes a $10 thermometer from a hardware store. Hold the thermometer in the return air grille (where air is pulled into the system) and note the temperature. Then hold it directly at a supply vent (where cold air comes out) and note that temperature. The difference is called the Delta T, or temperature split. A healthy, properly functioning central air system should produce a Delta T of 14 to 22 degrees Fahrenheit. If the air coming out of your vents is 75 degrees and the air going in is 80 degrees, your Delta T is only 5 degrees, which means the system is delivering almost no cooling. If the Delta T is above 22 degrees, the system may have severely restricted airflow, often from a clogged filter or a partially frozen coil. This single test tells you more than most homeowners learn from an entire service call. A technician who checks the Delta T first is doing their job correctly. How to Diagnose a Frozen Evaporator Coil If the system has been running for hours without cooling, go to your indoor air handler and look for frost or ice on the refrigerant lines or the unit itself. Frosted lines are the tell. If you see ice, do this specific sequence: Turn the system fully off at the thermostat, not just to Fan Only. Running the fan across a heavily frozen coil before the ice is fully off adds strain and can cause water overflow as it melts too fast. Full off. Leave it for two to four hours. Put towels under the air handler if there’s visible ice buildup. After thawing, replace the filter, then turn the system back on. If it refreezes within a day or two, the cause isn’t just airflow restriction. Low refrigerant or a failing blower motor are next. Three things cause a freeze-up: restricted airflow from a dirty filter or closed vents, low refrigerant charge, or a blower motor that isn’t moving enough air volume even when running. Sometimes two stack on top of each other and the freeze happens faster than either cause alone would produce. The Outdoor Unit: Fan, Compressor, Contactor, and Capacitor Go outside while the system is calling for cooling. Two components should be running: the fan at the top of the condenser unit and the compressor. You’ll hear the compressor as a low hum or vibration from the bottom of the unit. If the fan is spinning but the compressor is silent, or neither is running while the indoor blower is on, you have an electrical failure on the condenser side. The two most common causes are the capacitor and the contactor relay. The capacitor stores charge and delivers the voltage boost needed to start and run both the compressor and the condenser fan. Most residential systems use a dual-run capacitor with three terminals: C (common), HERM (compressor), and FAN. When it fails, the fan may spin slowly or the compressor may hum without starting. A licensed tech tests this with a multimeter set to capacitance mode. The reading should be within 6 percent of the rated microfarad (MFD) value printed