Why is my AC running but not cooling?
By HVAC Record · Published June 13, 2026 · Updated June 13, 2026
When an air conditioner runs but does not cool, start with the safe checks — thermostat set to cool/auto, a clean filter, the breaker and outdoor disconnect, a clear outdoor unit, and open vents — then look for a frozen evaporator coil; if those are fine the cause is usually low refrigerant or a failing compressor, which needs a licensed professional.
Start with the safe checks you can do yourself
Before assuming the worst, rule out the simple causes. Confirm the thermostat is set to "cool" and "auto" (not "fan"/"on"), and lower the setpoint a few degrees to trigger a call for cooling. Replace the air filter if it is dirty — a clogged filter is one of the most common reasons a system runs but barely cools, because it starves airflow across the coil.
Then check power: a tripped breaker or the outdoor disconnect can leave the indoor blower running (warm air) while the outdoor condenser is off. Reset a tripped breaker once. Finally, walk to the outdoor unit and clear grass, leaves, and debris from the fins so it can release heat, and make sure indoor supply vents and returns are open and unblocked.
Two more quick ones: replace the thermostat batteries if it is battery-powered, since a dying thermostat can call for cooling intermittently or not at all, and check the condensate drain. Many systems have a safety float switch that shuts cooling off when the drain clogs and the pan fills — so a backed-up drain can look like a no-cool fault even though the equipment is fine. If clearing the drain and resetting the switch restores cooling, that was the cause.
Look for a frozen evaporator coil
If air is weak and you see frost or ice on the indoor coil or the copper refrigerant line, the coil has frozen — usually from restricted airflow (dirty filter or coil) or low refrigerant. A frozen coil cannot absorb heat, so the system runs without cooling.
The safe response is to turn the cooling off and let the fan run to thaw the coil, which can take a few hours, then replace the filter and restart. If it freezes again quickly, stop running it — continuing to run a system that keeps freezing can damage the compressor — and book a diagnostic.
When it's refrigerant or the compressor — call a pro
If the basics check out and the coil is not iced, the likely causes move into professional territory: a refrigerant leak, a failing compressor, a bad capacitor or contactor, or a control fault. Low refrigerant in particular is never "normal consumption" — it means a leak that has to be found and sealed, and handling refrigerant legally requires an EPA-certified technician.
These are not DIY repairs. Beyond the safety and electrical risk, opening the refrigerant circuit without EPA Section 608 certification is not permitted. At this point the right move is to get a qualified local company out for a proper diagnosis.
A common, less catastrophic version of this is a failed start capacitor or contactor: the outdoor fan may not spin, or the unit hums but the compressor will not start, and the fix is usually a few hundred dollars. A technician can confirm it quickly. The reason to still call rather than guess is that low-cool symptoms overlap — a weak capacitor, a leak, and a dirty coil can look alike from the thermostat — and only a meter and gauges tell them apart.
How to pick who you call
Choose a company on evidence, not ad position. Look for an EPA-certified, licensed and insured contractor, ask for the diagnosis and repair cost in writing, and get a second quote for any large repair. If the fix is expensive and the unit is old, weigh repair against replacement before committing.
HVAC Record's metro pages let you shortlist quickly: each shows a company's Google review volume, latest review date, source-refresh date, and phone/website availability, so you can line up two or three well-evidenced local options to call.
Questions
- Why is warm air coming out when the AC is on?
- Common causes are the thermostat set to "fan/on" instead of "cool," a tripped breaker or outdoor disconnect leaving the condenser off, a very dirty filter, or low refrigerant. Check the thermostat, breaker, and filter first; if warm air persists, call a professional.
- Should I add refrigerant myself?
- No. Low refrigerant means a leak, not normal use, and topping it off only masks the problem. Handling refrigerant also legally requires an EPA Section 608 certified technician, so this is a professional repair.
- How long should I let a frozen coil thaw?
- Turn cooling off, leave the fan running, and allow a few hours for the ice to melt before restarting with a clean filter. If it refreezes quickly, stop running the system and book a diagnostic to avoid compressor damage.