Work this list in order. It is sorted by how often each fault turns out to be the culprit on a real service call, and the first four cost nothing to check.
Before anything else, separate the two symptoms. Air coming from the vents that is not cold is a different problem from no air at all, and both get described as “my AC is not cooling” on the phone. Put your hand on a supply register, then put your hand on the copper lines outside. Weak air with a cold coil points at airflow. Strong air that is room temperature points at the outdoor half of the system.
Is the filter blocking the airflow?
More summer no-cooling calls end at the filter than at any other part. An air conditioner cools by moving indoor air across a cold coil, so if the air stops moving, the cooling stops with it, and the system starves itself in a loop: less airflow, colder coil, condensation freezing on the fins, less airflow still.
Pull the filter and hold it up to a light. If you cannot see light through it, that is your fault, regardless of the date you wrote on the frame. A 1-inch pleated filter in a house with a dog and open windows can close up in six weeks.
While the filter is out, check the rest of the air path. Closed or blocked supply registers in unused rooms, a return grille behind a sofa, and crushed flex duct in an attic all do the same thing to a system that a dirty filter does.
Is the thermostat set the way you think it is?
Three settings account for most of these calls:
- Mode. It has to read COOL. An AUTO-changeover thermostat sitting between setpoints will run the fan and heat nothing, cool nothing.
- Fan. Set to ON, the blower runs continuously whether or not the system is cooling, so warm air comes from the registers between cycles and the house feels worse. Set it to AUTO. This alone explains a large share of “it blows but it is not cold”.
- Setpoint and schedule. A smart thermostat left in an away or eco hold after a trip will let the house drift ten degrees before it calls.
If you want to rule the thermostat out completely, drop the setpoint five degrees below room temperature and listen. You should hear the indoor blower start, and within a minute the outdoor unit should follow. If the indoor unit starts and the outdoor unit stays silent, skip to cause four.
Is the evaporator coil frozen?
Go to the indoor unit and look at the larger of the two copper lines where it leaves the air handler, the one wrapped in black foam insulation. Frost or a sleeve of ice there means the evaporator coil is frozen, and a frozen coil cannot transfer heat. The system will run all day and the house will keep getting warmer.
The fix has two halves, and only the first is yours:
- Thaw it. Switch the system to OFF and the fan to ON. Running the blower over a frozen coil melts it in two to four hours. Do not chip at ice on a coil, and do not run the compressor while the coil is iced.
- Find out why it froze. Ice on a coil has exactly two causes: not enough air across it, or not enough refrigerant in it. If you thaw the coil, fit a clean filter, and it runs normally, the filter was the cause. If it freezes again within a day or two on a clean filter, the charge is low and you have a leak, which is cause seven.
Put a towel under the air handler before you thaw it. A coil holding a few pounds of ice produces more water than the condensate pan is designed to take at once.
Is the outdoor unit actually running?
Walk outside while the thermostat is calling for cooling. You should hear the compressor humming and see the fan on top spinning. Four things commonly stop it, and all four are yours to check:
The breaker. Central air usually sits on a dedicated 240V double-pole breaker. A tripped one can rest in a middle position that reads as on at a glance. Push it fully off, then back on. If it trips again immediately, stop and call an electrician or an HVAC tech.
The outdoor disconnect. There is a small box on the wall within sight of the condenser, holding either a pull-out block or a switch. Landscapers, painters and anyone working on the siding pull them and forget.
The condensate float switch. High-efficiency systems and most attic air handlers have a float switch in the drain pan that cuts the cooling call when the condensate line clogs. This is a common July fault: the system stops cooling, everything else looks fine, and there is water standing in the pan. Clearing the line usually brings it straight back.
The furnace or air handler switch. In split systems the indoor unit powers the control circuit. A blower door left unseated after the filter change, or a service switch flipped in the basement, takes the cooling with it.
Has the run capacitor or contactor failed?
If the outdoor unit has power but responds with a hum and no rotation, or the fan spins while the compressor stays silent, the run capacitor is the first suspect and the contactor is the second. The capacitor is the single most common failed part in residential cooling, and heat shortens its life, which is why these fail in August rather than April.
The symptom set is specific. A hum for a second or two then silence, a fan blade that will start turning if you nudge it with a stick through the grille and then keeps running, a clicking contactor that does not pull in. Any of those is a capacitor or contactor diagnosis.
The parts are cheap, roughly $20 to $60. The job is not a DIY job anyway: a capacitor stores a lethal charge for a long time after the disconnect is pulled, and the terminals sit next to line voltage in a cabinet with no insulation between you and them. Budget $150 to $350 installed for either part, and expect a competent tech to test both while the panel is off.
Is the condenser coil dirty?
The outdoor unit dumps the heat it collects indoors into the outside air, and it does that through the fins that wrap the cabinet. Cottonwood, grass clippings, dryer lint and dust mat into those fins over a couple of seasons. A blanketed condenser raises head pressure, cuts capacity, and eventually trips the compressor on its internal overload.
This one you can do. Kill power at the disconnect, not just at the thermostat. Rinse the fins from the inside out with a garden hose on a normal spray, never a pressure washer, which folds the fins flat and does permanent damage. Clear two feet of space on every side and remove anything leaning against the cabinet.
If the fins are already combed flat over a large area, or the coil is packed solid, a proper chemical coil cleaning is a service call rather than a hose job.
Is the refrigerant low, and what does that mean for the compressor?
Refrigerant is not fuel. It circulates in a sealed loop and is not consumed, so a system that is low on charge has a leak, full stop. The telltales are a coil that refreezes on a clean filter, a hissing or bubbling sound at the indoor unit, and a temperature split across the coil that is too small: measure the air going into the return and the air coming out of a supply register, and expect a difference of roughly 16 to 22°F on a correctly charged system.
This is regulated work. The EPA requires that technicians who “maintain, service, repair, or dispose of equipment that could release refrigerants into the atmosphere must be certified”, and that includes adding refrigerant to an appliance. Beyond the legality, a top-up without a leak search is money poured into the atmosphere, and an overcharged system floods the compressor with liquid and destroys it.
Expect $500 to $1,600 for a leak found, repaired and recharged, depending on where the leak is and how much refrigerant the system holds. When the leak is in the evaporator coil itself, the repair often costs more than half of a replacement system, which is the point at which the conversation changes.
Behind all of this sits the compressor. Running a system with a low charge or a blocked condenser is what kills them, and a compressor replacement runs $1,300 to $2,900. On a unit past twelve years, that number is usually the end of the machine rather than a repair.
What normal cooling actually looks like
Knowing the healthy behavior tells you whether you have a fault or an expectation problem:
- The thermostat calls. The indoor blower starts within a few seconds.
- The outdoor unit starts within a minute, compressor and fan together.
- The large copper line outside becomes cold and sweats. The small line stays warm.
- Air from a supply register measures 16 to 22°F colder than air at the return.
- On a design-temperature afternoon the system runs most of the hour. That is correct, not a fault. Short cycling in the heat is the real warning sign.
- Water drips steadily from the condensate line outside. A system that produces no condensate on a humid day is not doing its job.
Repair or replace, in 2026
Under ten years old, repair. A capacitor, a contactor or a condenser fan motor on a system that age is straightforward money.
Past twelve to fifteen years, price the repair against a replacement before you approve anything over about $1,200, and pay attention to the refrigerant. Systems built for R-410A are getting more expensive to service as that refrigerant is phased down in favor of lower-global-warming replacements, and that gap widens every season. Worth knowing too: ENERGY STAR itself has stepped back from this category, and EPA “sunset the ENERGY STAR specification for central air conditioners” on 1 February 2026, so a contractor quoting an “ENERGY STAR certified” new system in 2026 is quoting from an old script. Compare SEER2 ratings and the federal minimum for your region instead.
Whatever you replace it with, insist on a load calculation rather than a like-for-like swap. An oversized system cools the air fast, shuts off before it removes any humidity, and leaves you with a cold clammy house and a compressor that short cycles itself to death.