AC Not Cooling: Why Your Air Conditioner Runs But Doesn’t Cool

Your air conditioner is running. The compressor is on, the fan is blowing, the thermostat is calling for cool. But the house won’t get below 78°F no matter how long it runs.

Running but not cooling is a specific failure pattern with a clear diagnostic order. The good news: most causes are either free to fix or very inexpensive — and only a few require calling a technician.

Work through these checks in order. Stop when you find the problem.

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Check 1: Thermostat settings (2 minutes, $0)

Before doing anything else, verify the thermostat:

• Is the system set to COOL, not HEAT or FAN ONLY? A thermostat accidentally set to HEAT produces heat; one set to FAN ONLY circulates uncooled air. Both feel like the AC isn’t cooling because they’re not.
• Is the setpoint below the current room temperature? If you set 72°F and the room is 72°F, the thermostat won’t call for cooling. Set it 3–4 degrees below current temperature to force a call.
• Is FAN set to AUTO, not ON? With FAN ON, the air handler blows continuously even when the compressor isn’t running — the air coming out between cooling cycles will feel warm. Switch to AUTO.

If these are all correct and the house still won’t cool: continue.

If you see an unfamiliar thermostat symbol — a snowflake icon, leaf icon, or alert triangle — see Nest thermostat symbols explained or your thermostat’s model guide to confirm the system mode.

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Check 2: Air filter (5 minutes, $5–25 for a new filter)

A clogged air filter restricts the airflow across the evaporator coil. With reduced airflow, the coil can’t absorb heat from the air efficiently — and eventually the coil freezes, blocking airflow entirely. The system runs but cools nothing.

1. Find the filter (typically in the return air duct or in a slot on the air handler/furnace).
2. Hold it up to a light source. If you can’t see light through it, replace it.
3. After replacing, wait 30–60 minutes before evaluating cooling performance — give the system time to reach steady state.

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Check 3: Indoor and outdoor unit — is everything running?

At the thermostat or air handler: set a cooling call. Within 2–3 minutes, you should feel air moving from the supply vents.

At the outdoor unit: the condenser fan (top of the unit) should be spinning, and you should hear the compressor running (a lower, consistent hum). If the fan isn’t spinning or no compressor hum is present:

• Fan not spinning, compressor humming: the condenser fan motor or its capacitor has failed. A capacitor is a $15–40 part. See HVAC Capacitor: Symptoms and Replacement.
• Nothing running (no hum): check the disconnect (gray box next to the outdoor unit) and the circuit breaker for the AC. A tripped breaker is a quick fix; a breaker that trips again immediately indicates a deeper electrical problem.
• Compressor hums but won’t start: failed run capacitor. See the capacitor guide above.

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Check 4: Look for ice on the indoor unit or refrigerant lines

Ice on the evaporator coil or on the copper refrigerant lines coming out of your air handler is a clear sign of a frozen coil. When the coil freezes:

• Airflow is blocked by ice
• The coil can’t absorb heat even when the compressor is running
• You may see ice forming on the line set (the insulated pipes going from the air handler to the outdoor unit)

If you see ice:

1. Turn off the AC immediately at the thermostat. Set to FAN ONLY or turn the system completely off.
2. Let the ice melt — this takes 1–4 hours depending on how much ice has accumulated. Don’t rush it with a hair dryer or heat gun.
3. After the ice has fully melted, replace the air filter (check #2) and try cooling again.
4. If the coil freezes again within a few hours: the cause is likely low refrigerant or a refrigerant restriction. Call a tech.

→ See Frozen Evaporator Coil: Why It Happens and How to Fix It for the full diagnostic.

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Check 5: The outdoor condenser coil

The outdoor unit (condenser) rejects heat from the refrigerant into the outside air. If the condenser coil is clogged with dirt, grass clippings, cottonwood, or other debris, it can’t reject heat effectively — the system’s cooling capacity drops, and your house stays warm.

Look at the condenser fins: the aluminum fins on the sides of the outdoor unit should be visible. If they’re clogged with debris, clean them:

1. Turn off power at the disconnect and breaker.
2. Remove the top fan guard (typically 4–6 screws).
3. Rinse from the inside out — direct water from inside the unit through the fins toward the outside. This pushes debris out. Do not spray from the outside in — you’ll pack debris deeper into the fins.
4. Let dry, reassemble, restore power.

Alternatively, a garden hose with moderate pressure on the outside fins will help if the debris is light and surface-level.

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Check 6: Temperature differential test

This test tells you how much the AC is actually doing:

1. Find a return air vent (where air is pulled INTO the system) and a supply vent close to the air handler (where cooled air comes OUT).
2. Measure the temperature at each with a thermometer.
3. The supply air should be 15–20°F colder than the return air in a properly functioning system.

What the reading tells you:

A delta-T below 10°F with everything else checking out (clean filter, no ice, outdoor unit running) is a strong signal that refrigerant charge is low.

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Check 7: The refrigerant — when to call a tech

If you’ve worked through checks 1–6 and the system is:

• Running (both indoor and outdoor units operating)
• Clean filter, no ice, condenser coil clean
• Temperature differential less than 10°F

…the most likely cause is low refrigerant charge.

Refrigerant doesn’t deplete in a closed system — if charge is low, there’s a leak. Diagnosing and recharging refrigerant requires:

• EPA 608 certification
• Manifold gauge set (to measure suction and discharge pressures)
• Refrigerant scale and proper refrigerant
• Leak detection equipment

This is not a DIY repair. Call a licensed HVAC technician.

A note on refrigerant adds: a technician who offers to “top off” the refrigerant without finding and fixing the leak is wasting your money. Low-refrigerant systems that are simply recharged without leak repair will be low again in one to three summers. Insist on leak detection.

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What to expect from a service call

When you call for AC not cooling, describe what you’ve already checked. This helps the tech narrow down quickly and often makes the service call faster:

• “Filter is new, no ice visible, both units are running” → tech goes straight to refrigerant pressures
• “Fan wasn’t spinning” → tech checks capacitor first

Likely outcomes:

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Quick-reference diagnostic summary

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FAQ

My AC worked fine last week. Can refrigerant just disappear? Not without a leak. Refrigerant in a sealed system doesn’t deplete with use — it cycles continuously. If refrigerant is low now but wasn’t a problem before, there’s a leak somewhere. A slow leak may have been gradually reducing charge over one to two seasons until it crossed a threshold.

It’s really hot outside (95°F+). Can the AC just be overwhelmed? Yes — if your system was sized for 90°F outdoor temperatures and you’re hitting 100°F, it may struggle to maintain 72°F indoors. This is a design-load issue, not a system failure. A functioning system should be able to maintain roughly 20°F below outdoor temperature; if it can’t, check all the basics first, then consider whether the system is properly sized for the load.

Can I run the AC if I see ice on it? Turn it off. Running the AC with a frozen coil can damage the compressor (liquid refrigerant slugging back to the compressor), and it’s not cooling anyway. Switch to FAN ONLY or OFF and let it thaw.

The AC was serviced last month. Why is it failing now? If it was working after service and failed on a hot day: a capacitor on its way out often passes when temperatures are moderate and fails when it has to work hardest in heat. Also check whether anything happened (power outage, storm, tripped breaker) right before it stopped cooling.