Start with what you can see
Is air coming out of the vents, and is it cold? That one question splits the problem in half. Cold air but no progress cooling the house points toward delivery, capacity or heat gain. Air that is not cold points at the refrigeration side. If little air is moving at all, see weak airflow.
The common causes
- Low refrigerant charge. Refrigerant is not consumed, so low charge means a leak. Cooling weakens gradually rather than stopping outright.
- Dirty condenser coil. The outdoor unit cannot reject heat through a layer of dirt, so the system runs and runs without getting ahead.
- A frozen evaporator coil. Ice blocks airflow, and the system can run continuously while delivering almost nothing.
- A failing capacitor or fan motor. The system appears to run while a component is not doing its job.
- Duct losses. Cold air escaping into an attic or crawl space never reaches your rooms.
- Undersized or poorly matched equipment, particularly if it has never kept up on the hottest days.
What you can check
The filter, the thermostat setting, and whether the outdoor unit is running and clear of debris. If you see ice anywhere, turn the cooling off and leave the fan running so it can thaw, then get it looked at rather than running it again.
When to call or text
If it is not the filter or the thermostat, it needs an assessment. A $139 diagnostic visit establishes which of the above you actually have.
Common questions
Why is my AC running constantly but not cooling?
Most commonly low refrigerant from a leak, a dirty condenser coil, a frozen evaporator coil, or a duct system losing the cold air before it reaches your rooms.
Should I turn the system off?
If you see ice, yes. Switch cooling off and leave the fan running to thaw it, then have it looked at. Running an iced system can damage the compressor.
Could it just be a dirty filter?
It can be, and it is worth checking first because a badly clogged filter restricts airflow enough to freeze a coil.
How much does it cost to find out?
$139 for a diagnostic visit, waived when you book the repair.
Reviewed and updated September 2026. Prices shown are owner supplied typical ranges, confirmed after an assessment. If anything here looks out of date, call (615) 400-4993 and tell us.