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Diagnostic guide

AC Blowing Warm Air

Your air conditioner runs but the air from the vents is warm — work through the causes from cheapest to most expensive.

System
Cooling
Causes covered
8
You can check
3 without a technician

Stop and read this first

  • A burning smell or scorching at the outdoor unit means shut the breaker off and call immediately.
  • Never open the electrical compartment of a condenser — capacitors hold a lethal charge.
  • In extreme heat with vulnerable occupants, treat loss of cooling as urgent.
  • If ice is on the refrigerant lines, shut cooling off before investigating further.

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. Thermostat set to Heat or Fan-only mode

    CommonYou can check
    How to check
    Display shows Heat, or the fan is set to On instead of Auto. Setpoint is above room temperature in Cool mode.
    The fix
    Set mode to Cool, fan to Auto, and setpoint five degrees below room temperature. Replace thermostat batteries if the display is dim.

  2. Tripped breaker or blown disconnect fuse

    CommonYou can check
    How to check
    The indoor blower runs but the outdoor condenser is silent. Check the panel breaker and the disconnect box near the outdoor unit.
    The fix
    Reset the breaker once. If it trips again, stop — a repeatedly tripping breaker indicates a fault that needs diagnosis.

  3. Failed run capacitor

    CommonNeeds a pro
    How to check
    Outdoor unit hums but the fan does not spin, or the compressor does not start. Unit is over five years old.
    The fix
    A technician replaces the capacitor — typically $150 to $400 installed. Do not open the unit yourself.

  4. Dirty air filter restricting cooling

    CommonYou can check
    How to check
    Filter is loaded. Airflow at vents is weak. Coil may have frozen previously.
    The fix
    Replace the filter. If the coil is iced, shut cooling off and run fan-only for two to four hours to thaw.

  5. Low refrigerant from a leak

    CommonNeeds a pro
    How to check
    Cooling worked last season but is weak now. Ice on the suction line. Hissing at connections.
    The fix
    A technician finds and repairs the leak, then recharges. Leak repair plus recharge runs $500 to $1,800 depending on location.

  6. Failed contactor

    PossibleNeeds a pro
    How to check
    Outdoor unit does not energize at all when cooling is called. Indoor blower runs normally.
    The fix
    A technician replaces the contactor — typically $150 to $350 installed.

  7. Reversing valve stuck (heat pump)

    PossibleNeeds a pro
    How to check
    You have a heat pump and warm air comes from vents in cooling mode. System may also blow cool air in heating mode.
    The fix
    A technician diagnoses and replaces the reversing valve — typically $800 to $2,000. On units over 12 years, compare against replacement.

  8. Failed compressor

    UncommonNeeds a pro
    How to check
    Outdoor unit hums loudly then trips the breaker. Little or no temperature difference across the coil.
    The fix
    Compressor replacement runs $1,500 to $3,500. On equipment over 10 years, full system replacement at $4,500 to $8,500 is often better economics.

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.

Air Conditioner Repair cost guideRepair or replace calculatorEstimate repair costAnalyze a quote

What the fix costs

Tools for this decision

Other symptoms