Quick answer:
An AC that isn’t cooling usually comes down to one of a few issues: restricted airflow (dirty filter or blocked outdoor unit), thermostat settings or dead batteries, a tripped breaker or safety switch, a clogged condensate drain, a frozen indoor coil from poor airflow or low refrigerant, or a failed start component like a capacitor. Start with simple checks you can do safely, then call a qualified technician if the system still won’t cool or keeps shutting off.
What are the most common reasons an AC stops cooling?
When cooling stalls, the system is almost always fighting one of three enemies: poor airflow, incorrect control signals, or a refrigerant/electrical fault. Understanding which bucket your problem falls into helps you act faster and avoid making things worse.
Airflow roadblocks
A clogged return filter chokes the blower, reducing the air crossing the evaporator coil. Without enough warm air moving over that coil, the coil can drop below freezing and ice over, which further blocks airflow. Outside, grass clippings or leaves pressed against the condenser coil keep heat from escaping. The result is lukewarm air from the vents and a system that runs and runs without meaningfully dropping the indoor temperature.
Controls and settings
The thermostat may be set to “Heat,” “Fan,” or “Auto” in a way that confuses the mode you want, or its batteries are dying. Some thermostats have a built-in compressor delay to protect the system that pauses cooling for a few minutes after a mode change. A tripped float switch triggered by a clogged condensate line can also stop cooling even though the thermostat appears to be calling.
Refrigerant and electrical faults
Low refrigerant from a leak reduces heat absorption, so you feel weak or warm air. Electrical issues like a failed capacitor, seized condenser fan motor, bad contactor, or a blown fuse keep key components from starting or staying on. These require specific tools and training to diagnose safely.
What can you safely check before you call for AC repair?
A few quick steps can rule out the easy stuff and sometimes get you back up and running without a service call:
- Replace or rinse the return filter if it’s visibly dirty, and make sure every supply and return vent is open.
- Set the thermostat to Cool, Auto fan, and a setpoint at least 5°F below room temperature; replace the thermostat batteries if present.
- Check your electrical panel for a tripped AC breaker. Fully turn it Off and back On once. If it trips again, stop and call a pro.
- Gently clear leaves, cottonwood fluff, and grass from the outdoor coil. Keep about 2 feet of open space around the unit.
- Look for water at the indoor unit or a full condensate pan. If a drain is clogged, switch the system Off and call for service.
- If frost or ice is present on the refrigerant lines or air handler, turn the system Off and set the fan to On for 2–3 hours to thaw. Don’t chip ice because you could damage the coil.
If these checks don’t restore cooling or if the system starts and stops repeatedly, it’s time for professional diagnostics.
Why is the thermostat set correctly but the AC still won’t turn on?
Thermostats can be sneaky culprits. Batteries that are almost dead may still power the screen but fail under load when signaling the system. Miswiring from a recent renovation can leave the cooling call unconnected. Many smart thermostats have minimum compressor off-time delays. So, if you’ve been cycling modes, wait five minutes and try again. Finally, the system may be honoring safety. These aren’t “glitches” — they’re protecting your equipment from damage.
Why does the AC run but blow warm air?
If the blower runs and you feel airflow, the unit is trying, but heat isn’t moving out of the house. Check the outdoor fan. If it isn’t spinning, heat isn’t being rejected outside. A failed fan motor or capacitor can be to blame. If the fan spins but the air is still warm, the compressor may not be engaging (bad capacitor or contactor), or refrigerant charge is too low to move heat efficiently. Both scenarios need a technician to test pressures, electrical components, and controls.
Can a clogged drain line shut off cooling?
Yes, and it’s by design. Most air handlers include a float switch in the secondary pan or drain. When algae or debris clogs the condensate line, the float rises and opens the safety circuit, shutting off cooling to prevent water damage. If you see water around the unit or the thermostat shows cooling but the system won’t start, this could be the cause. Don’t keep resetting it. Instead, clear the clog and flush the line so the safety can reset properly.
What is a bad capacitor and how would you know?
Capacitors store and release energy to help motors start and run. When a start/run capacitor fails, you may hear a faint hum from the outdoor unit with no fan movement. Sometimes the top of the capacitor bulges or leaks oil. Because capacitors carry a charge even with power off, replacement is not a DIY job. A tech will discharge and test the part safely and check companion components like the contactor while on site.
Is it safe to keep running the AC when it’s not cooling?
It’s tempting to leave the fan running “just for air,” but it can make problems worse. Low airflow or low refrigerant can cause the evaporator coil to freeze. In fact, continued operation risks liquid refrigerant returning to the compressor — an expensive failure. If the system isn’t cooling after your basic checks, switch it off at the thermostat and call for service. If you discover ice, thaw it with Fan On before the technician arrives. That’s because the system can’t be properly diagnosed while frozen.
How long should a cooling cycle last in normal conditions?
There’s no single “right” runtime because load varies with temperature, humidity, and sun exposure. Generally, expect steady, moderate-length cycles during peak heat. Short cycling (very brief on/off cycles) suggests oversizing, a failing capacitor, or a control fault. Very long cycles that barely move the temperature can indicate undersizing, low refrigerant, a dirty coil, or severe airflow restriction. Runtime patterns are a valuable diagnostic clue. Make sure to mention what you’re seeing when you call.
How do seasonal tune-ups prevent “AC not working” emergencies?
Preventive service is less about polishing covers and more about measurable performance. A thorough tune-up includes cleaning indoor/outdoor coils for proper heat transfer, verifying blower speeds and static pressure for airflow, checking refrigerant charge by superheat/subcool methods, inspecting electrical connections and component health, testing safeties and drains, and confirming thermostat calibration. Catching a drifting capacitor, a slowing fan motor, or early condensate growth turns a future no-cool call into a scheduled, low-stress fix.
Why does low refrigerant cause weak cooling and icing?
Refrigerant absorbs heat at the evaporator and releases it at the condenser. When the system is undercharged due to a leak, pressure and temperature relationships shift. The evaporator can fall below 32°F, freezing moisture in the air onto the coil. Ice blocks airflow, the coil gets even colder, and the system spirals into a freeze-up. Adding refrigerant without finding and repairing the leak is a bandage. Unfortunately, the problem will return and could damage the compressor. A competent repair includes leak detection, repair, evacuation, weighing in the correct charge, and verifying performance.
What if the outdoor unit runs but the indoor blower doesn’t?
No airflow across the evaporator means no sensible cooling, even if the outdoor unit is working. The indoor blower motor or control board may have failed, the filter or coil may be severely clogged, or the blower door safety switch might be open. Because indoor units are tied into safeties and high-voltage circuits, let a pro test the motor windings, capacitor (if used), and control signals rather than guessing.
Could ductwork be the reason some rooms won’t cool?
Absolutely. Even a perfectly charged and controlled AC can’t overcome poorly designed or leaking ducts. Undersized returns, crushed flex runs, disconnected branches, or leaky plenums starve rooms and waste energy. With the fan running, hold a tissue to a supply vent. See if you feel a strong, even flow. Then check the return. Do you feel a steady pull? A duct inspection and static pressure test can reveal restrictions that a filter swap won’t solve.
What are the telltale signs you need professional AC repair right now?
Some symptoms point to problems that can escalate quickly or risk safety:
- Burning or electrical odors, smoking components, or repeatedly tripping breakers
- Ice on refrigerant lines or the air handler that returns after thawing
Loud grinding, screeching, or metal-on-metal sounds from indoor or outdoor units
Shut the system off and schedule service. These conditions can damage the compressor, blower motor, or control board if ignored.
How do you choose between an AC fix and full replacement?
A smart decision balances condition, reliability, and efficiency — not just today’s problem. If the system is newer and the failure is minor, repair is usually the right call. If the unit is near or beyond its typical lifespan, suffers frequent breakdowns, or has a major component failure (compressor, coil) with other looming issues, replacement may offer better comfort, lower energy use, and fewer surprises. A good contractor will share test results, outline options clearly, and never pressure you either way.
Can AC repairs void your equipment warranty?
They can if performed incorrectly. Manufacturers often require licensed installation, approved parts, and documented procedures. DIY refrigerant handling is illegal and dangerous. Work with a qualified contractor who follows specs, documents readings, and provides clear invoices. That paper trail protects your warranty and your system.
What should you tell your AC technician to speed up the repair?
A few details help your tech hone in on the fault quickly. Tell them what the thermostat was set to, how long the system runs before shutting off, any noises or odors, whether you saw ice or water, recent power outages or renovations, and any error codes on a smart thermostat. Let them know what you already checked, and the results. Good information shortens diagnostic time and gets your comfort back faster.
Does running the fan only help when it’s not cooling?
Fan-only mode recirculates air without cooling. It can help thaw a frozen coil before service, but it won’t remove heat or humidity long term. If you rely on fan-only while the underlying problem persists, you risk additional damage. Use it briefly to thaw ice, then schedule repair.
Why does the breaker trip when the AC starts?
Air conditioners draw more current on startup. A weak capacitor, failing compressor, shorted wiring, or a degraded breaker can push current above limits and trip protection. Repeated resets are not a fix and can be dangerous. A technician will measure inrush current, inspect wiring and connections, and load-test components to find the real cause.
How can you keep “AC not working” problems from coming back?
Keep up with filter changes, give the outdoor unit breathing room, set reasonable temperatures, and schedule preventative maintenance before peak season. If your tech recommends a corrective action like adding a return duct, fixing a leak, or replacing a marginal capacitor, tackle it sooner rather than later. Small improvements compound into better comfort and fewer emergencies.
What DIY fixes should you avoid with modern AC systems?
Anything involving refrigerant, live electrical parts, or sealed components should be off-limits. Puncturing a coil to “let out pressure,” swapping capacitors without discharge tools, or adding the wrong refrigerant blend can turn a simple repair into a major replacement. Leave sealed-system and high-voltage work to licensed professionals; it’s safer, and it protects your warranty.
If your air conditioner isn’t keeping up or won’t start at all, contact Bud’s Plumbing, Heating, & Air. Our certified technicians deliver thorough diagnostics, clear recommendations, and repairs backed by our Legacy Guarantee. When comfort can’t wait, count on the team that’s built on experience, reliability, and a steadfast commitment to quality. Schedule your AC repair appointment now!