Prove thermostat demand, indoor airflow and outdoor-unit operation before considering refrigerant or electrical components. Check the size, orientation and condition of the filter and confirm that the registers are open. Look at the outdoor unit from a distance to see whether the fan turns, and clear leaves and debris around it with the power off.
What this covers
- Residential central air-conditioning and heat-pump systems in cooling mode
What it does not cover
Differential
Likely causes of “Air conditioner runs but does not cool the house”, and how to tell them apart
- 1
The filter or the indoor airflow is restricted
What separates it The air from the vents is weak or only slightly cool, the indoor coil may ice up and the filter is gray or clogged.
How to confirm Check the size, orientation and condition of the filter and confirm that the registers are open.
- 2
The outdoor unit is not running
What separates it The indoor blower blows air that is not cool, and the outdoor fan and compressor are silent or the outdoor unit is buried in leaves.
How to confirm Look at the outdoor unit from a distance to see whether the fan turns, and clear leaves and debris around it with the power off.
- 3
The thermostat setting or mode is wrong
What separates it The system runs the fan only, with the thermostat set to Fan, Auto or Off, or the setpoint is close to the room temperature.
How to confirm Set the thermostat to Cool and a few degrees below the room temperature and wait a few minutes.
- 4
Low refrigerant, a frozen coil or a failed component
What separates it Ice on the indoor coil or the refrigerant line, oily residue near fittings, or a humming outdoor unit that does not start.
How to confirm These need a technician; do not add refrigerant or chip ice from a coil.
Before you touch a setting
Record this first — changing settings destroys the evidence
- The indoor and outdoor temperatures, the thermostat setting and how long the system has run.
- Whether the outdoor fan and compressor run, and any ice or water around the indoor unit.
- The filter size and how long ago it was changed.
Try in this order
0 of 4 completedCapture the exact state
ReversibleRecord indoor/outdoor temperature, filter, blower and outdoor fan/compressor sound.
Separate the likely fault domains
ReversibleCheck power once, filter size/orientation, open registers and exterior debris with power off.
Correct only the proven, accessible condition
ReversibleRestore airflow or settings; arrange service for a silent/humming unit, ice, oil staining or warm supply air.
Verify once before going farther
ReversibleConfirm steady cool supply air and falling indoor temperature.
Stop boundary
When this stops being a self-service repair
- !Turn the system off and call a technician for ice on the coil, a silent or humming outdoor unit, or oil around refrigerant fittings.
- !Do not add refrigerant yourself; it requires certification and a leak repair first.
Evidence ledger
Sources behind this answer
Exact questions
Common questions
My air conditioner runs all day but the house is warm. Is it broken?+
Not necessarily. On a very hot day a system can run continuously and still lose ground. A drop of about 15 to 20 degrees Fahrenheit across the system is often cited as normal; a much smaller drop points at airflow or refrigerant.
How often should I change the air-conditioner filter?+
Follow the filter's label, commonly every one to three months in use, and more often with pets or dust.
Why does the AC run but not cool?+
Common causes are a dirty filter, low airflow, a frozen coil or a refrigerant issue.
What can I check safely?+
Replace a dirty filter, make sure vents are open and check that the outdoor unit is running. Look for ice on the indoor coil.
When do I call a technician?+
If the unit runs with clean filters and clear vents, the problem may be refrigerant or a component, which needs a licensed technician.