Check these yourself, in this order
None of these require tools or opening anything sealed. Work down the list before calling anyone.
- Thermostat set to COOL, and the fan set to AUTO rather than ON. Fan on ON runs the blower continuously, including between cooling cycles, which feels exactly like "running but not cooling."
- Air filter. A loaded filter starves the coil and is the single most common cause. If it is grey, replace it.
- Outdoor unit. Is the fan on top actually spinning? If the unit hums but the fan sits still, that is usually a failed capacitor — a small, inexpensive part.
- Outdoor coil. Cottonwood, grass clippings, and dust mat onto the fins. If you cannot see through it, it cannot reject heat.
- Breakers. Air conditioners typically have two — one for the indoor air handler, one for the outdoor condenser. The outdoor one can trip on its own.
If you see ice, stop
Ice on the refrigerant line or on the indoor coil means turn the system off at the thermostat and leave the fan running to thaw it. Do not keep running it in cooling.
A frozen coil sends liquid refrigerant back to the compressor, and compressors are built to compress vapor, not liquid. That is how a cheap airflow problem turns into the most expensive repair on the system. Thawing takes a few hours; a slugged compressor takes a new system.
What is left after that
If the filter is clean, the outdoor fan spins, and the coil is clear, you are into the parts that need a meter: refrigerant charge, capacitor, contactor, or the compressor itself.
One thing worth knowing before anyone quotes you: "low on refrigerant" is not a diagnosis. A sealed system does not consume refrigerant. If it is low, there is a leak, and adding more without finding it means paying again next season. Ask where the leak is.
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