Short answer
When an air conditioner runs but doesn't cool, check the thermostat setting, the filter, the outdoor unit's fan, the breaker, the coil for ice and the condensate drain. If the outdoor fan isn't spinning or the lines are iced, shut the cooling off to protect the compressor and call a technician.
1. Thermostat and filter
Most "broken" air conditioners are set to heat, set to fan "on" instead of "auto", or starved of air by a clogged filter.
- Confirm the mode is Cool and the setpoint is below room temperature.
- Set the fan to Auto. With the fan on, it blows room-temperature air between cycles.
- Pull the filter. If light can't pass through it, replace it.
2. The outdoor unit
Go outside while the system is calling for cooling.
- The top fan should be spinning and warm air blowing up.
- If it hums without the fan turning, shut the system off. That's often a failed capacitor, and running it can overheat the compressor.
- If it's completely silent, check the breaker and the disconnect switch by the unit.
- Look for cottonwood fluff or grass matted on the fins. Rinse gently with a hose after switching power off.
3. Ice on the lines or coil
Frost on the large copper line or ice on the indoor coil means too little airflow or low refrigerant. Switch the cooling off, run the fan for a few hours to thaw it, and replace the filter. If it freezes again, low refrigerant from a leak is likely, and that needs a technician.
4. The condensate drain
Many systems have a safety float switch that shuts the AC down when the condensate drain backs up. Look for water in the pan under the coil or around the furnace. Clearing the drain line can bring cooling back.
When it's just too hot outside
On a 100°F Kansas City afternoon, most systems are designed to hold about 75°F indoors, not 68°F. If the air at the vents is 15 to 20 degrees cooler than the air at the return, the system is probably working and simply at capacity.
When to call a technician
- The outdoor fan doesn't spin or the unit hums without starting.
- The coil ices again after thawing and a new filter.
- A breaker trips when the AC starts.
What the technician does: see ac repair.
Common questions
How cold should the air from my vents be?
Typically 15 to 20°F cooler than the air going into the return. Less than that points to an airflow, charge or component problem.
Can I add refrigerant myself?
No. Refrigerant handling requires EPA 608 certification, and low refrigerant means a leak that should be found.
Editorial note: this guide is general information for Kansas City-area homeowners, not a diagnosis. Conditions vary by home, so a licensed HVAC technician should confirm the cause before any repair.