AC Not Cooling
Check the filter and the outdoor unit before you call anyone.
Compiled by DIY Repair Guide · reviewed 2026-09-05 ·how we research these
When an air conditioner stops cooling, the cause is far more often maintenance than mechanical failure. A clogged filter alone accounts for a large share of no-cool calls, and technicians routinely arrive, change a filter, and charge a call-out fee for it.
The checks below are worth doing yourself first, in order. After them is what a technician will look at, and what it costs when the fault is real.
The Free Checks, in Order
Ten minutes here resolves a meaningful proportion of no-cool problems.
- Thermostat. Set to Cool, not Fan, and set below room temperature. Fan on Auto, not On. Fan set to On blows uncooled air continuously between cycles, which feels exactly like a broken air conditioner.
- Filter. Pull it out and hold it to a light. If you cannot see light through it, that is very likely your entire problem. A blocked filter starves the system of airflow, which drops capacity and eventually freezes the coil.
- Breakers. There are usually two. One for the indoor air handler, one for the outdoor condenser. Check both, and push each fully off then on.
- Outdoor unit. Is the fan spinning? Is the unit smothered in grass clippings, leaves or cottonwood? Before you touch it, pull the disconnect on the wall beside the unit and switch off its breaker. The condenser runs on 240 volts and holds a charged capacitor. With the power off, clear at least two feet around it and hose the fins gently from the inside out.
- Vents and returns. Closed registers and furniture over a return starve airflow the same way a dirty filter does.
- Condensate drain. Many systems have a float switch that shuts cooling off when the drain pan fills. A clogged condensate line is a common and cheap cause of a system that simply will not run.
If the indoor fan runs but the outdoor unit is silent, that is a strong signal. Power to the condenser, a failed capacitor, or the float switch. If neither runs, look at the thermostat and the breakers first.
The Frozen Coil Trap
A very common sequence: the filter clogs, airflow drops, the evaporator coil gets too cold, moisture on it freezes, and the ice blocks airflow completely. Now the system runs constantly and blows air that is barely cool.
Look for ice on the refrigerant lines at the outdoor unit, or on the coil inside the indoor cabinet. Ice usually means restricted airflow rather than a broken compressor.
To clear it, switch the system off but leave the fan running. Fan-only circulates room air over the coil and clears a light freeze in one to three hours. A coil that has been icing for days can take most of a day, so do not conclude the fix has failed at hour four. Never chip at the ice. The tubing and fins are thin, they puncture easily, and a punctured coil turns a filter problem into a refrigerant repair. While it thaws, change the filter and open every vent.
If it freezes again within a day or two of a clean filter and clear vents, the cause is usually low refrigerant, which means a leak and a technician.
What a Technician Will Find
When the free checks come up empty, these are the usual diagnoses, roughly in order of how often they come up.
| Fault | What you notice | Typical repair cost |
|---|---|---|
| Failed run capacitor | Outdoor fan silent or humming | $150–400 |
| Dirty condenser coil | Runs constantly, cools weakly | $100–250 (cleaning) |
| Clogged condensate line | System will not start at all | $100–250 |
| Low refrigerant / leak | Ices up, weak cooling, gradual decline | $300–1,500+ |
| Failed contactor | Outdoor unit will not engage | $150–400 |
| Blower motor failure | Little or no air at the vents | $400–900 |
| Compressor failure | Hums, trips the breaker, no cooling | $1,500–3,000, often replace |
| Full system replacement | — | $5,000–12,000 |
Refrigerant Is Not Consumed
This matters because it is widely misunderstood, and the misunderstanding is expensive. An air conditioner is a sealed system. It does not use up refrigerant the way a car uses fuel. If it is low, there is a leak.
A recharge without finding the leak is a temporary purchase. The same charge escapes again, often within a season. If you are told the system needs topping up, ask where the leak is and what fixing it costs. On an older system with an expensive leak, replacement often makes more sense than repeated recharges.
There is a further cost issue for older equipment. Systems using R-22, phased out in the US, face very high refrigerant prices where it is available at all. On an R-22 system a refrigerant repair usually argues for replacement.
Handling refrigerant requires Environmental Protection Agency (EPA) Section 608 certification in the US. It is not a homeowner job, and the sealed side of an air conditioner is not something to open.
Repair or Replace
Central air conditioners last roughly 12 to 18 years. The usual rule of thumb is to weigh the repair cost against the age of the unit. A large repair on a system past about twelve years is money spent on equipment that is already near the end.
| Situation | Lean toward |
|---|---|
| Under 8 years, any repair | Repair |
| 8–12 years, repair under ~$1,000 | Repair |
| 8–12 years, repair over ~$1,500 | Get both quotes |
| Over 12 years, major repair | Replace |
| Compressor failure out of warranty | Usually replace |
| R-22 system needing refrigerant work | Usually replace |
What Prevents Most of This
Nearly every no-cool call traces back to airflow, and airflow is maintenance.
- Change the filter every one to three months during the cooling season. Monthly with pets or allergies. This is the single highest-value thing you can do.
- Keep two feet clear around the outdoor unit and rinse the fins each spring.
- Flush the condensate line annually. A cup of distilled vinegar down the access tee keeps the algae growth that blocks it in check.
- Leave vents open. Closing registers to "save energy" raises duct pressure and reduces total airflow.
- Book a service before the season, not during it. Rates and wait times both rise on the first hot week.
Common questions
Why is my AC running but not cooling?
Most often a clogged filter restricting airflow, a frozen evaporator coil, a dirty outdoor condenser, or low refrigerant. Check and change the filter first. It is free and it is the most common single cause.
Why is my AC frozen?
Almost always restricted airflow, usually a dirty filter or closed vents, sometimes low refrigerant. Switch the system off and run the fan alone for one to three hours to thaw it, change the filter, then run it again. Refreezing after that points to a refrigerant leak.
How much does it cost to recharge AC refrigerant?
Commonly $300 to $1,500 depending on the refrigerant type and how much is needed, and considerably more for R-22. But refrigerant is not consumed. If it is low there is a leak, and a recharge without a leak repair is temporary.
Why is my outdoor AC unit not turning on?
Frequently a failed run capacitor or contactor, both relatively inexpensive parts. Also check that the outdoor unit has its own breaker and that it has not tripped, and look for a nearby disconnect switch that may have been turned off.
How often should I change my AC filter?
Every one to three months during the cooling season, and monthly with pets, allergies or a dusty environment. A blocked filter is behind a large share of no-cool service calls.