Why Your AC Isn’t Cooling: A Thermostat-First Checklist

When the AC “isn’t cooling,” the fault is somewhere on a chain that runs thermostat, then call for cooling, then compressor, then refrigerant, then airflow, then your room. Start at the thermostat end, because it’s free, it takes a minute, and it’s the most common culprit. Then follow your exact symptom down to the specific fix.

First, 60 seconds at the thermostat

Before you call anyone or touch the outdoor unit, run through this checklist. Eight checks, under a minute.

1. Mode is COOL, not Heat, Auto, or Off. Auto mode on some thermostats switches between heating and cooling based on the schedule, and it can land on Heat if the weather was cold recently. Verify the word “COOL” is displayed, not just an icon.

1. Setpoint is below the current room temperature by at least 2°F. If the room is 76°F and you set 76°F or higher, nothing will run. That’s correct behavior, not a malfunction. Drop the setpoint to at least 74°F and wait 60 seconds for the system to respond.

1. Fan mode is AUTO, not On. With the fan set to On, the blower runs continuously whether the compressor is firing or not. Between cooling cycles, it circulates unconditioned air and makes the house feel like the AC is broken. Switch to AUTO, and the fan will only run when the system is actively cooling.

1. Display is active, not blank or dimmed. A thermostat in deep sleep or with a locked screen can look like it’s on when it isn’t. Press any button to wake it and confirm the mode and setpoint are actually displayed.

1. Battery or power is good. Low batteries cause intermittent behavior, not always a dead screen. The thermostat might power on and show a setpoint but fail to send a full cooling signal to the air handler. Replace the batteries if they’re more than a year old before troubleshooting anything else.

1. “Wait” or delay indicator is gone. After a power blip or a breaker trip, most thermostats impose a 3 to 5 minute compressor lockout to protect the refrigerant system from a hard restart. The screen will show “Wait,” a timer, or a similar message. This is normal. Let it clear before concluding anything is broken.

1. Schedule has no vacation setback active. A vacation hold set to 85°F will override your 74°F setpoint until you clear it. Check the schedule menu for any hold, override, or vacation program that may be suppressing normal operation.

1. WiFi thermostat: confirm it is not in Hold or Away mode. Smart thermostats controlled via an app can be put into Away or Hold modes that override the local setpoint. Check the app as well as the physical display; the app sometimes shows a different active setpoint than the screen.

If all eight check out and the system still isn’t cooling, find your symptom below.

Match your symptom to the fix

It won’t turn on at all, or says “Wait” / “AC Power Loss”

A thermostat delay after a power interruption is deliberate and typically clears in under 5 minutes. If the wait indicator is gone but the system still won’t fire, the issue may be an actual power failure at the unit, a tripped float switch in the condensate drain pan, or a failed contactor at the outdoor unit. A tripped float switch is common in humid climates and is DIY-resettable once the drain is cleared.

• AC not working, thermostat says “Wait” · What “Wait” means
• Honeywell “AC Power Loss” message

There’s no “Cool” option on the screen

The thermostat isn’t configured for cooling, or the wiring or jumper is missing. This happens after a DIY thermostat swap where the installer skipped the Y wire or didn’t enable the cooling stage in setup. It also happens on heat-only systems where someone installed a multi-stage thermostat without checking compatibility.

• No Cool option on Nest · ecobee not showing Cool · Honeywell no Cool option

It’s running but blowing warm or hot air

The system is calling for cooling, but the refrigerant circuit isn’t doing its job. Two quick checks from outside: is the outdoor unit fan spinning? If the fan is stopped but the compressor is humming, the run capacitor is likely failed. If both the fan and compressor are running, check the suction line (the larger insulated pipe at the outdoor unit). That line should be cold and sweating. If it’s iced over, the system has been running low on airflow or refrigerant for a while. Either way, that’s a technician call. On heat pumps, a stuck reversing valve will send hot air into the house even when the thermostat calls for cooling.

• Nest blowing hot air on Cool · Amazon/smart thermostat blowing hot on Cool · New Honeywell blowing hot on Cool
• Outdoor unit dead or fan not spinning? Can you run AC without the condenser fan?

It’s blowing cool air but never reaches the setpoint

A central AC system can typically cool a house to about 20°F below the outdoor temperature on the hottest days. On a 100°F afternoon, expecting 74°F inside is asking the system to hit a 26°F split, which is past what most residential equipment can deliver. If the outdoor temperature is extreme and the house is close but not quite there, that may be the ceiling.

If the system held 74°F last summer but cannot this summer under similar conditions, refrigerant loss is the most likely cause. Charge doesn’t evaporate on its own; there is a leak somewhere. A dirty filter or a frozen evaporator coil can also cap cooling capacity, so check the filter first before calling a tech.

• AC won’t go below 74°F · ecobee not cooling to set temperature
• Refrigerant charge: high superheat / low subcooling and the low-superheat / high-subcooling explainers
• No condensation and no cooling

Outdoor unit runs, but no air comes from the vents

The compressor is running but the indoor blower is not moving air. Check the air handler breaker separately. Central AC systems typically use two breakers: one for the outdoor condenser and one for the indoor air handler. They are labeled separately in the panel and can trip independently. A tripped air handler breaker leaves the outdoor unit running while producing no airflow inside. Reset it once; if it trips again immediately, there is an electrical problem in the air handler that needs diagnosis.

• Outside unit running but no air blowing inside
• ecobee says “no equipment running” but the fan is on

It cools, then shuts off too soon and the room swings

That’s short cycling, and it has its own fix list.

• AC short cycling: causes & fixes

When to stop and call a pro

DIY-safe: thermostat mode and settings, battery replacement, replacing a clogged filter, clearing a condensate float switch, hosing off a dirty condenser coil. Stop and call a pro for anything involving refrigerant (a low charge, frozen coils that keep coming back after defrost), a failed capacitor or contactor, or line-voltage work at the disconnect or air handler electrical panel. Those aren’t settings problems, and guessing at them gets expensive.

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Frequently Asked Questions

Why is my AC running but not cooling the house?

A: The most common causes are a thermostat set to Fan On instead of Auto (circulating unconditioned air between cycles), a low refrigerant charge (the compressor runs but the refrigerant circuit can’t absorb heat properly), a failed outdoor fan capacitor (the compressor runs but the condenser fan doesn’t, so the system can’t reject heat), or a dirty filter or frozen evaporator coil blocking airflow. Start with the thermostat fan setting, then check the filter. If both are fine and the outdoor unit sounds normal but the air inside is warm, low refrigerant is the next suspect.

How do I know if my AC is low on refrigerant?

A: You can’t measure charge without gauges, but there are reliable signs: the supply air from registers is only slightly cooler than room temperature (instead of the normal 14 to 20°F split), the suction line at the outdoor unit is frosted or iced over, the system runs continuously without reaching setpoint, and the house held setpoint last season under similar conditions but can’t this season. Any of these patterns warrants a service call. A technician needs to locate and repair the leak before recharging; adding refrigerant without fixing the leak is a temporary patch.

Why won’t my AC cool below 75°F?

A: On a hot day, there’s a physics ceiling: residential central AC is typically sized to maintain about 20°F below outdoor temperature. On a 97°F day, the lower bound is roughly 77°F. If the outdoor temp is moderate and the system still can’t reach 75°F, the likely causes are a dirty evaporator or condenser coil, low refrigerant charge, or an undersized system for the square footage. Check and replace the filter first, then have the coils cleaned if that doesn’t help.

My AC was working yesterday. Why isn’t it cooling today?

A: Sudden changes usually point to a single component failure rather than a gradual problem. Check the thermostat settings first (nothing changed accidentally during the night), then check both breakers in the panel. If the outdoor unit is running but there’s no cool air, a run capacitor may have failed overnight; capacitors are the most common single-component failure on outdoor units and are inexpensive to replace. If the outdoor unit is completely silent, check for a tripped breaker or a blown fuse at the outdoor disconnect box.

How long should it take for AC to cool a house?

A: A properly sized and charged system should drop the indoor temperature roughly 1°F every 15 to 20 minutes under normal conditions. A house that is 10°F above setpoint should reach setpoint in about 2.5 to 3.5 hours on a moderate day. If the system runs for more than 4 hours without getting close to setpoint on a day below 95°F outdoors, something is wrong: low refrigerant, a dirty coil, a filter choked with debris, or the system is undersized for the space.

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Thinking it might be time to replace it?

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Related: Best Thermostat Settings for Summer · AC Short-Cycling · Is it humidity, not heat?