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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.

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.

Estimate the repair costAnalyze a quote

What the fix costs

Tools for this decision

Other symptoms