AC Not Cooling? 10 Most Common Causes & Fixes

Q: Why is my AC running but not cooling?

The most common causes are: dirty air filter, low refrigerant charge, dirty condenser coil, frozen evaporator coil, or a faulty compressor. Start by checking the filter and condenser airflow, then check superheat/subcooling.

Q: How do I know if my AC is low on refrigerant?

Signs of low charge: warm supply air, high superheat, low subcooling, frozen suction line, hissing sounds, or higher-than-normal evaporator temperature. Verify with manifold gauges and a superheat/subcooling calculation.

Q: Can a dirty filter cause AC to stop cooling?

Yes. A clogged filter restricts airflow across the evaporator coil, which reduces heat transfer and can cause the coil to freeze. Once frozen, the AC stops cooling entirely. Always check airflow first (ABC: Airflow Before Charge).

Updated July 2026 · By FieldCalc HVAC · 10 min read

"The AC is running but the house won't cool down." It's the most common service call in HVAC. This guide walks through the 10 most likely causes — in the order you should check them — so you can diagnose and fix the problem fast. Remember the golden rule: ABC — Airflow Before Charge. Always verify airflow before touching refrigerant.

The Diagnosis Checklist (In Order)

Follow this sequence. Most no-cooling calls are solved in the first three steps.

Dirty Air Filter

The #1 cause. A clogged filter starves the evaporator of air, reducing heat transfer. In severe cases, the coil freezes and cooling stops entirely.

Fix: Replace the filter. Check that supply and return airflow feels balanced. If the coil is frozen, let it thaw completely before restarting.

Dirty or Blocked Condenser Coil

The outdoor unit can't reject heat if the coil is clogged with dirt, grass, or debris. Head pressure rises, capacity drops, and the system may trip on high-pressure safety.

Fix: Clean the condenser coil with a garden hose and coil cleaner. Trim vegetation at least 2 feet away from the unit.

Low Refrigerant Charge (Leak)

If airflow is fine but the system still underperforms, check charge. Low charge means a leak — systems don't "consume" refrigerant.

Fix: Connect gauges, check superheat and subcooling. If charge is low, find and repair the leak before adding refrigerant. Never just "top off" — it will leak again.

Frozen Evaporator Coil

Caused by low airflow (dirty filter, weak blower) or low charge. Ice insulates the coil, blocking heat transfer. If you see frost on the suction line, the coil is likely frozen.

Fix: Turn off cooling, switch fan to ON to thaw. Check filter, blower motor, and evaporator coil cleanliness. Then verify charge. See our evaporator coil freezing guide for details.

Faulty or Misadjusted TXV / EEV

The metering device controls refrigerant flow into the evaporator. If it sticks closed, the evaporator starves (high superheat). If it overfeeds, the compressor can slug liquid (low superheat).

Fix: Check superheat against target. If superheat is way off and charge is correct, the TXV bulb may be loose, the valve may be restricted, or the sensing bulb is poorly clamped.

Bad Capacitor

The run or start capacitor weakens over time. The compressor or fan may hum but not start, or run sluggishly. A common failure on hot days.

Fix: Test capacitance with a multimeter. If reading is more than 10% below the rated value, replace. Always discharge capacitors before handling.

Weak or Failed Contactor

The contactor switches power to the compressor and condenser fan. Pitted or burnt contacts cause intermittent operation — the unit may short-cycle or not start at all.

Fix: Inspect contacts for pitting/burning. Check for 24V at the coil. Replace if contacts are worn or the coil isn't pulling in.

Reversing Valve Leak (Heat Pumps)

In heat pumps, a leaking reversing valve lets high-side gas bleed into the low side, reducing capacity in both heating and cooling. The valve may also be stuck between positions.

Fix: Check temperature difference across the valve body. If discharge gas is bypassing, the valve needs replacement.

Compressor Failure

The compressor may be shorted to ground, open winding, or mechanically seized (valve plate failure). The unit hums but doesn't pump, or trips the breaker immediately.

Fix: Check winding resistance and continuity to ground. If windings are open or shorted, the compressor must be replaced. For mechanical failure, compare amp draw and discharge temperature to specs.

Undersized or Aging System

If all mechanical checks pass but the space still won't reach setpoint, the system may be undersized, ductwork may be leaky, or the unit is at end of life with degraded capacity.

Fix: Measure delivered CFM vs load calculation. Check duct integrity. If the unit is 15+ years old, capacity may have dropped 10-20% — consider replacement.

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Quick Diagnostic Flowchart

Step	Check	If Problem → Go To
1	Filter dirty?	→ Cause #1 (replace filter)
2	Condenser blocked?	→ Cause #2 (clean coil)
3	Coil frozen?	→ Cause #4 (thaw, check airflow)
4	Superheat/subcooling off?	→ Cause #3 (leak) or #5 (TXV)
5	Compressor not running?	→ Cause #6 (cap) or #7 (contactor)
6	All checks pass, still warm?	→ Cause #10 (size/age)

Frequently Asked Questions

Why is my AC running but not cooling?

The most common causes: dirty filter, low refrigerant, dirty condenser, frozen evaporator, or a faulty compressor. Always check airflow first (ABC: Airflow Before Charge).

How do I know if my AC is low on refrigerant?

Warm supply air, high superheat, low subcooling, frozen suction line, hissing sounds, or higher evaporator temperature. Verify with gauges.

Can a dirty filter cause AC to stop cooling?

Yes — it starves the evaporator, reduces heat transfer, and can freeze the coil. Always check the filter first.