The system is running, air is moving, and the house is not getting cool. Work through the following in order. The first several checks cost nothing, take about fifteen minutes total, and resolve a meaningful share of the calls that come into our office during a July afternoon. Nobody at a repair company minds telling you this, incidentally. A technician driving across Pinellas County to flip a breaker back on is a waste of your money and our afternoon, and we would rather you found it yourself.
Start at the top and do not skip ahead.
Start at the thermostat
Confirm three things: the mode is set to COOL, the setpoint is genuinely below the current room temperature, and the fan is on AUTO rather than ON.
That last one accounts for more confused calls than it should. With the fan switched to ON, it runs continuously whether or not the compressor is cooling, so you feel air from the vents and reasonably conclude the system is working. It is only half working. Air moving with no cooling behind it feels warm and, worse, re-evaporates moisture off the coil and blows it back into the house, which makes the place feel more humid than doing nothing at all.
Replace the batteries if it takes them. Dying thermostats behave erratically long before they go fully dark, and battery symptoms mimic almost every other fault on this page.
Location matters too. A thermostat sitting in afternoon sun, above a lamp, or on a wall backed by a hot chase reads several degrees higher than the actual room, so it runs the system past the point of comfort while never satisfying itself. Bad placement is a fixable problem, and our thermostat troubleshooting guide covers how to tell.
Check the filter, then look for ice
Pull the filter and hold it up to a light. Cannot see through it? You have very likely found your answer.
Clogged filters restrict airflow across the evaporator coil. Less air across the coil means the coil gets colder than designed, and once its surface drops below freezing the condensation on it turns to ice. Ice blocks airflow further, which makes more ice, and within a few hours the coil is a solid block and the system has stopped cooling entirely while the blower keeps humming along as if nothing is wrong.
If you find ice on the refrigerant lines or on the coil itself, stop and do this: switch the system to fan-only, leave it that way for several hours until everything has thawed completely, then install a clean filter and restart. Do not chip at the ice, and do not run the compressor to melt it faster. Running a frozen system risks sending liquid refrigerant back to the compressor, and compressors are not built to compress liquid. That failure is the expensive one.
Ice that returns after a filter change means something else, usually low refrigerant or a blower problem, and at that point it is a service call.
Look at the outdoor unit
- Is the fan on top actually spinning? A hum with no rotation is the classic signature of a failed run capacitor, which is a common and relatively minor failure, though the capacitor stores a charge and is genuinely dangerous to touch. Leave it alone
- Are the fins packed with grass clippings, leaves, cottonwood or dryer lint? A blocked condenser cannot reject heat, and a system that cannot reject heat cannot absorb any either
- Is there roughly two feet of clear space all the way around, and nothing sitting on top of it?
- Is the disconnect switch in the box beside the unit still pushed in? Landscapers and pressure-washing crews pull those and forget
Check both breakers
Residential air conditioning almost always runs on two circuits: one feeding the air handler indoors, one feeding the condenser outside. Homeowners check the panel, see the air handler breaker sitting fine, and conclude the electrical side is healthy.
A tripped condenser breaker produces exactly the symptom you are describing. Air moving, nothing cold.
Reset it once, and understand that a breaker rarely trips for no reason. Should it hold, watch the system for the rest of the day. Should it trip again, stop there and call someone. Repeated tripping signals a real fault, most often a failing compressor drawing too much current or a short somewhere in the circuit, and resetting it over and over is how a repairable problem becomes a fire.
The Florida-specific cause
Here is the one that catches people who moved from elsewhere. Your air conditioner pulls gallons of water out of the air on a humid day, and all of it drains away through a small PVC line. That line clogs with algae and biological slime, the water backs up, and a float switch in the drain pan senses the rising level and shuts the system down on purpose to keep your ceiling from getting soaked.
Depending on the model, the float switch either kills the whole system or just the cooling while leaving the fan running.
Look for standing water in the drain pan under the air handler, or a puddle nearby. Most drain lines have an accessible cleanout, a capped vertical section near the air handler, and a wet vacuum applied to the outdoor end of the line will pull the blockage clear in about a minute. Our guide to condensate problems walks through it in detail. This is also the single most preventable failure on the list, since a routine maintenance visit includes clearing and treating that line.
What is left needs a technician
If you have gone through everything above and the house still will not cool, the remaining causes all require gauges, meters, and a license.
- Low refrigerant, which always means a leak, because refrigerant is not consumed. Adding more without finding and repairing the leak just puts the same money into the atmosphere on a slower schedule. More on that in what low refrigerant actually means
- A failed capacitor or contactor. Both are inexpensive parts and both carry enough stored or live voltage to seriously injure you
- A failing compressor, which is the expensive scenario, and the point at which system age starts driving the repair-versus-replace decision rather than the fault itself
- A metering device or TXV fault, which mimics low refrigerant closely enough that undertrained technicians add charge to compensate, which makes everything worse and can flood the compressor
Cool but the house still feels wrong
Different problem entirely. If the thermostat reads what you set it to but the air feels clammy and heavy, nothing is broken. The system is satisfying the temperature too quickly to spend time removing moisture, which points at oversizing or an airflow problem rather than a fault, and no repair will fix it because there is nothing to repair.
Related: AC repair and diagnostics, emergency service, and common problems explained.
