AC Running but Not Cooling: What to Check, In Order

The system runs. You can hear the outdoor unit and feel air at the registers. The house just won’t cool. Before you assume the worst, understand the order of causes: airflow problems and a $20 electrical part account for a big share of these calls, and refrigerant is further down the list than most homeowners think. Work it in this order.

The walk-around a tech does first — no tools

Before any of the deeper checks, do the pass a tech does in the first two minutes. It needs no tools and no gauges, and it sorts most of these calls. One condition first: let the system sit OFF for a while before you start, so any frost on the coil has melted. You want to watch it start from a settled, idle state — not frosted up.

Turn it to COOL and drop the temperature so it calls. Then go outside to the condenser — the box with the fan — and read three things: the fan, the air, and the pipes.

• Fan spinning, air blowing HOT: the outdoor unit is doing its job. The problem is inside — airflow or the indoor side; work the checks below.
• Fan spinning, air blowing but NOT hot — or blowing cold: the fan runs but the compressor probably is not. The fan alone moves air; it does not make cold.
• Fan NOT spinning, but you hear humming or sputtering and feel some heat: that noise is refrigerant moving through the compressor — the compressor is alive and the outdoor fan is the failure. The usual suspects are the capacitor (the cheaper, easier fix) or wiring — these boxes live outside, water gets in, and connections go bad. This is the capacitor check, step 5 below.
• The pipe test: two copper lines run into the unit, a small one and a big one. Feel the BIG pipe. It should be cold — cold and wet, sweating. Give it a few minutes and it should get colder and start to condensate. A big pipe that stays room-temperature says the charge may not be right, or the airflow is blocked: a bad filter, a dirty coil, any of the checks below. You are not touching anything electrical here — just watching and feeling a pipe.
• Total silence — nothing spins, hums, or gets warm: that is a power problem until proven otherwise. A tripped breaker is the first look. Behind it can be a grounded compressor, or a capacitor that grounded out and took the circuit with it.

If everything outside checks out and the house still will not cool, come back inside to the thermostat and filter (steps 1 and 2) — and know the last resort on this pass: if there is no power to the furnace or air handler, a hardwired thermostat with a common C wire will usually be dead too. A battery thermostat can look perfectly alive on a system that has no power at all.

1. Thermostat settings and batteries

Battery-powered stats go stupid at low battery — they’ll hold a setpoint that isn’t real or drop the schedule. Replace the batteries, confirm mode is COOL, and set the fan to AUTO, not ON. Fan ON runs the blower all the time and re-evaporates the condensate off the coil, which makes the house feel clammy and warm. Check for a schedule or hold override that’s fighting you. Free fix.

2. The filter

Pull the filter. If you can’t see light through it, it’s strangling the system. A starved coil gets cold enough to freeze. Look for ice on the copper lines at the outdoor unit, or frost on the indoor coil through the air-handler access. Thaw procedure: turn the system off at the thermostat, set fan to ON to move air, and give it 2–4 hours, or just shut it down overnight with the filter out. Do not run it with ice on the coil — liquid refrigerant can slug the compressor. New filter $5–$20. What the result tells you: if it cools after a thaw and a fresh filter, the filter was the whole problem.

3. The outdoor unit

Stand at the condenser. Is the fan on top actually spinning? If it hums and never turns, jump to check 5. If it spins, look at the coil — St. Louis cottonwood and grass clippings mat the outside face fast. STOP: shut the power off at the disconnect before you touch anything in that cabinet. Pull the pull-out block or flip the breaker; the capacitor and line voltage are still live otherwise. Rinse the coil gently from the top down with a garden hose. No pressure washer — you’ll bend fins and drive dirt deeper. Coil cleaner is $15–$25 if it’s greasy. Keep two feet of clearance around it.

4. Registers and return airflow

Walk every room. Closed supply registers, a rug over a return grille, furniture against a return, a clogged filter grille — all of it cuts airflow and can freeze the coil even with a clean filter. Open the ones you closed and vacuum the return grilles.

5. Capacitor failure — the number-one sudden death, and the dirty coil behind it

If the unit was cooling an hour ago and now it just hums, strains, or sits silent, the first suspect is the dual-run capacitor — one can that runs both the compressor and the fan on most condensers. Signs: the fan is slow to start, the unit hums and trips the breaker, or the compressor clicks off after a few seconds. The part itself is $15–$45; a capacitor service call runs $150–$300 total in most markets.

The safety sequence, in order: pull the outdoor disconnect AND switch the 240-volt breaker off inside. Then prove the power is dead with a meter or a non-contact tester before your hands go in the cabinet. After the disconnect there shouldn’t be power — but people wire things creatively, and “shouldn’t” is not a test. STOP: a capacitor holds a lethal charge even with the power off. It must be discharged properly before terminals are touched. If that sentence is new to you, this is where DIY ends and the $150–$300 call is the smart money.

If you do swap it: the old can’s top is usually rusted — photograph it before moving a single wire. The terminals are lettered: C is common, HERM goes to the compressor, FAN to the fan motor. Move the wires one at a time to the same letters on the new can. Universal replacements may show one extra terminal or jumper option — the letters still rule. Match the ratings printed on the old can (microfarads and voltage), and have the contactor checked while the cabinet is open.

The half of this job almost everybody skips — and the reason the capacitor died: clean the coil while you’re standing there. A condenser is a giant outdoor air filter; it inhales everything around it. With the power still off, hose the coil from the top down in slow left-right sweeps about as wide as each grille section and watch the dirt pour out the bottom. The worst setup in new construction: the dryer vent mounted right beside the condenser, blowing lint straight into the coil — a unit in that spot can need cleaning monthly. A matted coil can’t breathe, so head pressure and temperature climb, the compressor runs hot and inefficient, and the capacitor cooks. Blown capacitors are usually the first sign of a dirty coil, not the whole story — swap the can without cleaning and you’ll likely be back inside a season.

Button it up, restore power, and let it run 15–20 minutes to stabilize — that holds for R-410A and the newer refrigerants alike. Then the no-gauges verdict: the big suction line should be cold and sweating, and the air off the top fan should be warm. If it is, you’re tuned in about as far as eyes and hands can prove.

6. Low refrigerant — a leak, not a tune-up

If the filter is new, the coils are clean, registers are open, and the air coming out of the vents is only 8–12°F colder than room temperature (it should be 15–20°F colder), suspect charge. The pipe test from the walk-around is your no-gauge early warning: a system short on charge rarely gets that big line cold and sweating the way it should. Refrigerant does not get used up. Low means a leak. A legitimate repair is: find the leak with an electronic detector or nitrogen pressure test, fix it, evacuate the system, and weigh in the charge to the data plate. Topping it off and walking away is a waste of your money and, under Section 608 of the Clean Air Act, refrigerant may only be handled by EPA-certified technicians — venting is illegal. A leak search and repair on accessible joints runs $250–$600; refrigerant itself runs roughly $75–$150 per pound installed on an R-410A system, and a residential system holds about 4–15 pounds.

What a legitimate service call looks like

Expect $99–$189 for the diagnostic, often credited toward the repair. The tech should check filter and airflow, indoor and outdoor coils, capacitor and contactor, measure superheat and subcooling, check amp draw, and tell you what he found before he quotes anything. A “tune-up” that is only a filter swap and a hose rinse is not a diagnostic.

The maintenance that prevents the worst call: the condensate drain

Ask a tech what destroys the most property in this trade and the answer is the air conditioner — not by freezing or burning, but by leaking. All summer, your system pulls moisture out of the air and sends it down a condensate drain. That drain clogs about once a year with hair, rust, and whatever residue blows around in the air. When it clogs, the water backs up and pours down — typically right on top of the furnace control board sitting below the coil. It fries the board, ruins whatever is underneath, rots a wood floor, and rusts out the furnace and air handler. And because it happens in July, nobody connects it to the furnace that will not start in October.

The prevention is once a year and takes minutes. Go to the end of the drain line, put a shop vac on it, and suck the clog out. Then prove the line is clear: pour water in at the coil if you can reach it and watch it come out the end, or run the AC and confirm condensate builds and flows.

One trap — literally. Many drains include a water trap, a dip in the line that holds water to block air. Vacuuming the line can suck that trap dry, and an empty trap causes its own trouble. After you clean the line, the trap has to refill. Run the AC five to ten minutes and check the drain end: if you feel air being sucked through the line toward the furnace, the trap is empty and pulling air. Two fixes: plug the end of the line with your hand for a few seconds — you become the trap while the water rises and fills it — or simply turn the system off and let the water flow down and fill the trap on its own. Some installs have an access cap for refilling it directly. Next start, it should drain the way it was built to.

When the math says replace

If the system is 15 years or older, uses R-22, and needs a compressor or an evaporator coil plus a leak repair, the repair can hit 30–40% of a new system. At that point replacement is usually the better money. A full system in the St. Louis area in 2026 runs roughly $6,000–$12,000 installed depending on tonnage and SEER2. Run your numbers through the calculators on this site before you sign a repair estimate.

STOP: do not open the outdoor electrical panel or the indoor air-handler cabinet unless the disconnect and breaker are off. Line voltage and stored capacitor charge are both in there.