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Diagnostic guide

Air Conditioner Not Cooling

Work through the causes of an air conditioner that runs but does not cool, ordered from most to least likely.

System
Cooling
Causes covered
10
You can check
6 without a technician

Stop and read this first

  • A burning smell or visible scorching at the condenser means shut the breaker off and call immediately.
  • Never open the electrical compartment of a condenser. Capacitors store a lethal charge after power is removed.
  • If ice is present, shut cooling off. Running a frozen system pulls liquid refrigerant into the compressor and destroys it.
  • In extreme heat with vulnerable occupants, treat loss of cooling as an emergency and relocate rather than wait.

Work through these in order

Ordered from most to least likely. Working top down means you rule out the cheap, common causes before paying anyone to look at the expensive, rare ones.

  1. Dirty or clogged air filter

    CommonYou can check
    How to check
    Pull the filter and hold it up to a light. If you cannot see light through it, it is restricting airflow.
    The fix
    Replace the filter. If the coil has already frozen, turn the system to fan-only for two to four hours to thaw before restarting cooling.

  2. Failed run capacitor

    CommonNeeds a pro
    How to check
    The outdoor fan does not spin, or you hear a hum from the condenser without the fan starting.
    The fix
    A technician tests and replaces the capacitor. Do not open the unit yourself — capacitors hold a lethal charge even after power is disconnected.

  3. Tripped breaker or blown disconnect fuse

    CommonYou can check
    How to check
    Check the panel for a breaker sitting between on and off, and the outdoor disconnect near the condenser.
    The fix
    Reset the breaker once. If it trips again, stop — a repeatedly tripping breaker indicates a fault that needs diagnosis.

  4. Thermostat set or wired incorrectly

    CommonYou can check
    How to check
    Confirm the mode is Cool, the setpoint is below room temperature, and the fan is set to Auto rather than On.
    The fix
    Correct the settings and replace thermostat batteries. A fan set to On circulates uncooled air continuously and is often mistaken for a failure.

  5. Frozen evaporator coil

    CommonYou can check
    How to check
    Look for ice on the refrigerant lines at the outdoor unit or on the coil inside the air handler.
    The fix
    Shut cooling off and run the fan to thaw. Freezing is a symptom — the underlying cause is restricted airflow or low refrigerant, and it will recur until that is fixed.

  6. Dirty condenser coil

    CommonYou can check
    How to check
    Inspect the outdoor unit for grass clippings, cottonwood, leaves or matted fins.
    The fix
    Kill power at the disconnect and rinse the coil gently from the inside out with a garden hose. Never use a pressure washer — it flattens the fins.

  7. Low refrigerant from a leak

    PossibleNeeds a pro
    How to check
    Ice on the suction line, a hissing sound, and cooling that degrades gradually over seasons.
    The fix
    A technician locates the leak, repairs it, and recharges by weight. Refrigerant is not consumed in normal operation — if it is low, there is a leak, and simply adding more is not a repair.

  8. Failed contactor

    PossibleNeeds a pro
    How to check
    The outdoor unit does not start at all while the indoor blower runs normally.
    The fix
    A technician replaces the contactor. Pitted or welded contacts are a common failure on units over eight years old.

  9. Clogged condensate drain triggering a safety switch

    PossibleYou can check
    How to check
    Water around the air handler, or a system that shuts down on its own with no fault at the thermostat.
    The fix
    Clear the drain line with a wet vacuum at the outdoor termination. The float switch resets once the pan drains.

  10. Failed compressor

    UncommonNeeds a pro
    How to check
    A loud hum then a breaker trip, or a unit that runs continuously with almost no temperature difference across the coil.
    The fix
    A technician confirms with electrical testing. On a system past ten years, replacement of the system is usually the better economic decision.

Direct answer

When central AC runs but the house stays warm, the most common causes are a dirty filter restricting airflow, a failed run capacitor or contactor, or low refrigerant charge from a leak — not necessarily a dead compressor. Start with the filter and that the outdoor unit is running with the indoor fan on. If the coil ices up or the outdoor fan never spins, shut cooling off and schedule service; running a frozen or undercharged system can destroy the compressor.

Severity and urgency

Fix within days

Comfort and humidity control suffer quickly above 90°F, but the bigger risk is mechanical damage: low charge, restricted airflow, or a seized outdoor fan motor can turn a $250 capacitor job into a $2,000+ compressor replacement if you keep forcing cooling.

Cost of ignoring it

A neglected refrigerant leak forces the compressor to run hot and can fail within one heat wave ($1,500–$3,500 repair vs. $5,000–$12,000 replacement). Ice on the evaporator from a dirty filter can flood the compressor with liquid refrigerant on thaw. Mold risk rises when humidity stays high because the coil never dehumidifies properly.

How a pro will diagnose it

Expect a $99–$179 diagnostic fee that covers amp draws on the compressor and fan motors, capacitor testing, superheat/subcooling or pressure readings (EPA-certified tech), condensate and filter inspection, and thermostat call verification. A fair writeup names the failed part, whether the charge is low, and whether the leak is repairable before quoting parts. Ask if the diagnostic credits toward same-visit repair.

Prevention and maintenance

  • Replace 1" filters every 30–90 days (monthly in summer with pets or dusty work)
  • Keep outdoor condenser coils clean and maintain 12" clearance on sides, 24" above
  • Schedule annual cooling tune-up before peak season — includes amp checks and coil cleaning
  • Do not close more than 20% of supply registers; blocked returns cause freeze-ups
  • Address small refrigerant leaks when found; topping off without repair is temporary

For trade owners

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Common questions

Could dirty or clogged air filter be causing this?
Pull the filter and hold it up to a light. If you cannot see light through it, it is restricting airflow. Replace the filter. If the coil has already frozen, turn the system to fan-only for two to four hours to thaw before restarting cooling.
Could failed run capacitor be causing this?
The outdoor fan does not spin, or you hear a hum from the condenser without the fan starting. A technician tests and replaces the capacitor. Do not open the unit yourself — capacitors hold a lethal charge even after power is disconnected.
Could tripped breaker or blown disconnect fuse be causing this?
Check the panel for a breaker sitting between on and off, and the outdoor disconnect near the condenser. Reset the breaker once. If it trips again, stop — a repeatedly tripping breaker indicates a fault that needs diagnosis.
Could thermostat set or wired incorrectly be causing this?
Confirm the mode is Cool, the setpoint is below room temperature, and the fan is set to Auto rather than On. Correct the settings and replace thermostat batteries. A fan set to On circulates uncooled air continuously and is often mistaken for a failure.
Why is my AC running but not cooling?
Usually restricted airflow (filter, closed registers) or a failed electrical component (capacitor, contactor). Less often, low refrigerant from a leak. The outdoor unit must run when cooling is on; if only the indoor fan runs, the problem is at the condenser or in controls.
How long can I run AC that is not cooling?
A few hours while you change the filter is fine. If you see ice on the lines, hear grinding outdoors, or smell burning, shut it off. Running an iced or undercharged system damages the compressor — the most expensive part.
Is low refrigerant always a leak?
On a sealed system, yes — refrigerant does not get 'used up.' A tech should locate the leak before recharging. Small leaks at flare fittings or the evaporator may be repairable; coil leaks often push toward replacement.
Can a dirty filter alone stop cooling?
Yes. A loaded filter drops airflow enough that the evaporator temperature falls below freezing, ice forms, and effective cooling stops. Replace the filter, thaw with fan-only mode 2–4 hours, then retry.
What does AC repair typically cost when it is not cooling?
Capacitor or contactor: $150–$350 installed. Fan motor: $350–$650. Refrigerant leak repair plus recharge: $250–$900 depending on refrigerant type. Compressor: $1,500–$2,800 — often near replacement economics on units over 12 years.
Should I reset the breaker if the AC tripped?
Once, after the unit has been off five minutes. If it trips again, leave it off — repeated trips indicate a locked rotor, shorted wire, or failing compressor. Do not bypass the breaker.
Why would the thermostat show cool but air is warm?
The indoor fan can run without the outdoor unit if the contactor failed, high-pressure switch opened, or the thermostat lost its Y wire connection. Check that the outdoor fan and compressor actually start when cooling calls.
Is it normal for AC to struggle on the hottest day?
Systems are sized for design conditions, not every record high. They may run longer but should still drop temperature. If output is lukewarm all day, that is a fault — not normal overload.
When is AC not cooling an emergency?
Burning smell, loud grinding, smoke, or a tripping breaker warrant shutting down immediately. For vulnerable occupants without cooling in extreme heat, treat as urgent for health — otherwise schedule within a few days.
Repair or replace if the compressor failed?
Use the 5,000 rule: multiply equipment age by repair cost. If the product exceeds ~$5,000, replacement often wins. On R-22 legacy units, replacement is almost always the better financial choice.

Once you know what it is

Step one

Know what it should cost

Get the range before you call, so the first number you hear is not the only number you have.

Step two

Decide repair or replace

On equipment near end of life, the repair is usually the first of several.

Step three

Check the quote

Compare against the typical range and confirm the terms before you sign.

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What the fix costs

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