AC Not Cooling? Common Causes and What to Do

Start Here: What to Check Before Calling

Before spending $75–$150 on a diagnostic call, work through these checks in order. They resolve a surprising percentage of "not cooling" complaints — no tools required.

1. Check the thermostat. Confirm it is set to COOL (not HEAT or FAN), the temperature setpoint is at least 5 degrees below the current room temperature, and that it is not in a schedule or "hold" mode that is overriding your setting. If it runs on batteries, replace them.
2. Check the air filter. Pull the filter from the return air vent or air handler. If it is gray, clogged with dust and debris, or has not been changed in more than 90 days, replace it. A severely clogged filter starves the system of airflow and can cause the evaporator coil to freeze — leading to zero cooling.
3. Check the circuit breaker. Find your electrical panel and locate the breaker labeled for the AC or air conditioner. If it is tripped (in the middle position), flip it fully OFF and then back ON. If it trips again immediately, do not reset it again — a repeatedly tripping breaker indicates an electrical fault that a technician needs to assess.
4. Check the outdoor unit. Go outside and look at the condenser unit. Is the fan spinning? Is debris (leaves, grass clippings, shrubs) blocking the sides or top? The condenser needs clear airflow to expel heat. Clear any debris and ensure nothing is blocking the unit within at least 2 feet on all sides.
5. Check all vents and returns. Walk through the house and confirm supply vents are open and unobstructed by furniture or rugs. Confirm return air vents (larger grilles, typically in hallways or ceilings) are clear. Blocked vents create pressure imbalances that reduce cooling capacity.
6. Look for ice on the lines. Check the refrigerant lines running from your outdoor unit into the house (they look like copper pipes wrapped in insulation). If there is visible ice, turn the system OFF and switch the fan to ON to defrost. Do not restart in COOL mode until the ice has melted completely — usually 2–4 hours.

If none of these steps resolve the issue, the problem is internal to the system and requires a technician with diagnostic equipment.

Most Common Causes of AC Not Cooling

1. Low Refrigerant (Refrigerant Leak)

Refrigerant is the substance that absorbs heat from indoor air and transfers it outside. When refrigerant is low, the system cannot move enough heat to cool the space. Refrigerant does not deplete on its own — if your system is low, there is a leak somewhere in the system.

Signs of low refrigerant: air from vents is slightly cool but not cold, ice forms on the lines or outdoor unit, hissing or bubbling sounds near the unit, and electricity usage is higher than normal.

A contractor will find and repair the leak, then recharge the system with the correct refrigerant type. Cost: $200–$600 for a straightforward recharge; more if the leak is in the coil or requires coil replacement.

2. Dirty or Frozen Evaporator Coil

The evaporator coil (inside the air handler) absorbs heat from your home's air. If airflow is restricted by a dirty filter or blocked vents, the coil can freeze solid — blocking all airflow and stopping cooling entirely. Turning the system off to defrost will restore airflow, but the underlying cause (filter, vents, or low refrigerant) must be addressed to prevent repeat freezing.

A dirty coil that has not frozen may still reduce cooling efficiency significantly. Professional coil cleaning costs $100–$400.

3. Failed Capacitor

The capacitor helps start and run the compressor and fan motors. When it fails, the compressor may attempt to start, hum loudly, then shut off — leaving the fan running but the system not actually cooling. You might notice the outdoor unit fan is not spinning, or the compressor sounds labored.

Capacitor replacement is one of the most common and least expensive AC repairs: $150–$350 for parts and labor, often done same-day.

4. Dirty Condenser Coil

The condenser coil (in the outdoor unit) expels heat from the refrigerant to the outside air. When it is clogged with dirt, cottonwood fluff, or debris, it cannot release heat effectively. The result is a system that runs constantly but barely cools. Annual condenser coil cleaning is part of a standard HVAC tune-up.

5. Failed Compressor

The compressor is the heart of the cooling system — it pressurizes the refrigerant to drive the heat exchange process. A failed compressor means no cooling, period. Signs: the outdoor unit runs (fan spins) but the compressor makes no noise or makes grinding/clanking sounds.

Compressor replacement runs $800–$2,500. On systems older than 10 years, replacement of the full unit often makes more financial sense. See our repair vs. replace guide for the full decision framework.

6. Thermostat Malfunction

A faulty thermostat may fail to send the correct signal to start the compressor, may read room temperature incorrectly, or may have failed entirely. If your unit runs but the thermostat display is blank or behaving erratically, the thermostat is a likely culprit. Replacement costs $100–$300 including installation.

What the Contractor Will Do

When a technician arrives for a "not cooling" call, here is the typical diagnostic process:

• Visual inspection: Check thermostat settings, filter, visible refrigerant lines, outdoor unit condition, and disconnect box at the condenser.
• Electrical testing: Test voltage at the disconnect, check capacitors, contactors, and run a start test to see if the compressor and fan engage properly.
• Refrigerant pressure check: Using gauges, measure suction and discharge pressure to determine if refrigerant charge is within spec.
• Temperature differential measurement: Measure supply air and return air temperature. A healthy system produces a 16–22 degree Fahrenheit temperature drop. A narrow split indicates refrigerant, airflow, or coil issues.
• Quote and diagnosis: Present findings and a written repair quote before any work begins.

A good technician will explain what they found and why the recommended repair is the right solution. If a quote is vague or jumps straight to expensive repairs without walking through the diagnosis, ask for clarification or get a second opinion.

Emergency vs. Scheduled Service

Not every "not cooling" situation requires emergency dispatch. Consider the conditions:

Call for Emergency or Same-Day Service When:

• Outdoor temperatures are above 90°F and indoor temperatures are rising rapidly
• Elderly residents, infants, or people with heat-sensitive medical conditions are in the home
• Pets are in the home and cannot be moved to a cooler environment
• The system has completely stopped (no airflow at all, not just poor cooling)

A Next-Day Scheduled Appointment Is Fine When:

• Outdoor temperatures are mild (under 80°F) and the home is manageable overnight
• You have alternative cooling (window units, fans, or can stay elsewhere)
• The system is blowing some cool air, just not reaching the target temperature

Emergency and after-hours service carries a premium of $75–$150 over standard pricing. For non-urgent situations, a scheduled appointment avoids that extra charge.

For full pricing on what repairs cost once a technician diagnoses the problem, see our AC repair cost guide.

AC Not Cooling FAQ

Why is my AC running but not cooling the house?

The most common causes are: low refrigerant (from a leak), a frozen evaporator coil, a failed capacitor, a dirty condenser coil blocking heat transfer, or a faulty thermostat. Start by checking your thermostat, filter, and circuit breaker before calling a technician.

What should I check before calling an HVAC technician?

Check: (1) thermostat set to COOL and at least 5 degrees below room temperature; (2) air filter — replace if gray or clogged; (3) circuit breaker; (4) outdoor unit running and unblocked; (5) vents open and clear. These checks resolve many "not cooling" calls.

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

Signs include: the system blows barely cool or warm air, ice on the refrigerant lines or outdoor unit, hissing or bubbling sounds, and higher electricity bills. Refrigerant does not deplete on its own — low refrigerant means there is a leak.

Why is my AC freezing up?

A frozen AC is usually caused by restricted airflow (dirty filter, blocked vents) or low refrigerant. Turn the system off and switch the fan to ON to defrost for several hours before restarting. If it freezes again, call a technician.

Is AC not cooling an emergency?

It depends on conditions. High outdoor temperatures with elderly residents, young children, or medically vulnerable people in the home makes it an emergency. Mild weather with alternative cooling available allows for a next-day scheduled appointment.

How much does it cost to fix an AC that is not cooling?

Cost depends entirely on the cause. A dirty filter is free. A refrigerant recharge costs $200–$600. A capacitor replacement runs $150–$350. A failed compressor is $800–$2,500. See our AC repair cost guide for full pricing.

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