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Air conditioner running but not cooling
A system that runs but does not cool is usually short of airflow or short of refrigerant. One of those you can check yourself.
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The short version
- Most likely cause
- A clogged filter or a blocked outdoor unit choking the airflow.
- Safe to check
- The thermostat setting, the filter, and whether the outdoor unit is running.
- Call a technician when
- The air stays warm with a clean filter, or there is ice on the lines.
Safe to check yourself
- Confirm the thermostat is set to cool and below room temperature.
- Look at the filter and replace it if it is grey with dust.
- Check that the outdoor unit is running and not buried in leaves.
- Look for a tripped breaker for the air conditioner.
Stop and call
- There is ice on the copper lines or the indoor coil.
- The outdoor unit hums but the fan does not turn.
- The breaker trips again after one reset.
- The system has needed refrigerant added before.
What stops a running system from cooling
- Restricted airflowA dirty filter or dirty coils reduce airflow and efficiency. It is the first thing a technician checks, and the one part you can check too.
- Low refrigerantThe Department of Energy puts it plainly: a system low on refrigerant was either undercharged at installation or it leaks.
- A failed electrical partCapacitors, contactors and fan motors wear out. The system may hum, start and stop, or run only the indoor fan.
- A thermostat or sensor faultA thermostat in the wrong mode or reading the wrong temperature can keep a healthy system from cooling.
What a technician will do
- Check airflow: filter, coils and blower.
- Measure refrigerant pressures and temperatures.
- Test the electrical parts in the outdoor unit.
- Repair the cause. If refrigerant is low, find the leak before adding more.
On leaks, the EPA's advice to homeowners is to ask technicians to "locate and repair leaks instead of 'topping off' leaking systems."
Related guides
Ice on the air conditioner
What an AC capacitor replacement costs, and why
Air conditioner turning on and offSources
- US Department of Energy
- US Environmental Protection Agency: R-22 phaseout