Guide
AC Running But Not Cooling: How to Narrow It Down
A practical walkthrough of why an air conditioner runs without cooling, which checks are safe to do yourself, and which symptoms mean you should stop and call.
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An air conditioner that runs but does not cool is telling you that something is interrupting one of three things: airflow, refrigerant, or heat rejection outdoors. This guide helps you describe the problem accurately and rules out the causes you can safely address yourself.
Nothing here asks you to open refrigerant lines, add refrigerant, or work inside an electrical panel. Those tasks are unsafe and, in the case of refrigerant, legally restricted.
Start with the thermostat
Confirm the mode is set to cool, the setpoint is below the current room temperature, and the fan is set to auto rather than on. A fan left in the on position keeps blowing after the compressor stops, which makes room air feel warm and is often mistaken for a failure.
If the thermostat is battery powered, weak batteries can produce erratic behavior. Replace them before assuming the worst.
Check the air filter and vents
A loaded filter is the most common cause of poor cooling. Restricted airflow drops coil temperature until ice forms, and an iced coil cannot transfer heat at all, so the system runs endlessly while the house gets warmer.
Replace the filter, open all supply vents, and make sure furniture or rugs are not covering returns. If the coil has already iced, turning the system to fan only for a few hours will thaw it, and you should see water in the drain pan as it does.
Look at the outdoor unit
Go outside while the system is calling for cooling. You should hear the compressor running and see the top fan spinning.
- Fan not spinning while the unit hums: often a failed capacitor or fan motor. Shut the system off and call.
- Coil packed with grass, leaves, or cottonwood: heat cannot escape, so pressures rise and cooling drops.
- Unit completely silent with the thermostat calling: check the breaker and the outdoor disconnect.
- Ice on the copper line leaving the unit: airflow or charge problem, not something to run through.
Symptoms that mean stop and call
A breaker that trips repeatedly, a burning smell, loud grinding or screeching, water running out of the air handler, or ice that returns after a thaw all need a technician. Continuing to run the system through any of those can turn a repair into a replacement.
What a technician will measure
Temperature split between return and supply air, static pressure and airflow across the coil, refrigerant pressures and superheat or subcooling, amp draw on the compressor and fan, and capacitor values against their ratings. Those measurements distinguish a leak from an airflow problem from a failing compressor, which is why a diagnosis is worth more than a guess.
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