AC Not Working or Not Turning On? How to Find the Cause

Air Conditioner Not Working? Start With the Symptom

An AC not working can mean several different things, and each has its own short list of causes. Troubleshooting an air conditioner goes fastest when you name the symptom first, so pick the line that matches what you see.

• Nothing happens at all: AC not turning on
• It runs, but the air is not cold: AC not blowing cold air, which has its own page
• The outdoor fan is still, or no air comes from the vents: AC fan not spinning
• The air coming out is warm: AC blowing warm air
• Ice on the pipe or the coil: AC freezing up
• The breaker keeps tripping: AC tripping the breaker
• It starts, stops and starts again: AC short cycling
• A loud or new sound: AC making noise
• Water where it should not be: AC leaking water, which has its own page

Troubleshooting an AC unit this way works for any brand, and it is how a technician starts too. Most AC problems on this list, like most air conditioning problems anywhere, start with one of four cheap things: the thermostat, the filter, a breaker or a blocked drain. That is why every section below puts the free checks first.

AC Not Turning On

An AC unit not turning on is usually a power or control problem, not a broken machine. That holds whether the whole air conditioner is not turning on, the AC is not kicking on when the thermostat calls, or it stopped working overnight. Work from the thermostat outward, because the cheapest causes sit closest to your hand.

• Thermostat. Set it to cool and a few degrees below the room temperature. If the screen is blank, replace the batteries. Then wait: most thermostats hold the compressor off for about five minutes after any change, so an AC that will not turn on straight away may only be on its delay.
• Breakers. The air handler and the outdoor unit are on separate breakers. Reset a tripped one once. If it trips again, leave it off, because a breaker that will not hold is reporting a fault.
• The switch by the indoor unit. It looks like a light switch and gets turned off by accident.
• The outdoor disconnect. A small box on the wall beside the condenser. A pulled or loose handle there means no power outside.
• The float switch. A full drain pan trips it and it cuts the cooling on purpose. If the pan under the indoor coil holds water, clear the drain and the system will start again. The page on an AC leaking water covers that drain.
• The service door. A furnace or air handler panel left ajar opens a safety switch, and nothing runs until it is seated.

Those six cover an AC not coming on after a storm and an AC not kicking on when the house warms up. They also cover an air conditioner not turning on after the winter, an AC unit not working with the thermostat lit, or a home AC not working at all. An AC that stopped working mid-afternoon belongs here too.

If the heat is out as well, the fault is probably one the two systems share, such as the thermostat or the indoor unit's power, and the furnace not working guide covers the furnace side.

If all six are right and the AC won't turn on, the fault is inside the equipment: a blown low-voltage fuse on the control board, a failed transformer, a contactor or a capacitor. A capacitor or contactor is commonly a $150 to $450 repair.

AC Not Working, Thermostat Blank: What to Check

A blank screen changes the search, because now the thermostat is not asking for anything. If it runs on batteries, change them and the AC and thermostat not working together is solved in a minute.

With the thermostat blank, AC not working nearly always comes down to one of four causes. A thermostat with no batteries takes its power from the indoor unit, so a blank one means that power has been cut. Three things cut it on purpose: the float switch on a full drain pan, the door switch on a panel left ajar, and a small fuse on the control board that blows when a wire shorts. The fourth is the transformer that makes the low voltage, and replacing one runs $100 to $250. Check the pan, the door and the indoor breaker before anyone sells you a thermostat, because a new thermostat on a dead circuit is just as blank.

Outside AC Unit Not Turning On but Inside Is

This is the most common version: air comes from the vents, but it is not cold, and the outdoor unit is silent. The indoor blower and the condenser are two machines on two circuits, so one can run without the other.

It gets described as an outside AC not turning on, an outdoor AC unit not turning on, an air conditioner outside unit not running or simply the AC not running outside. Check the outdoor breaker and the disconnect first. Then look at the thermostat wiring if the thermostat was replaced recently, since a loose wire to the outdoor unit gives exactly this result. After that it is the contactor, which is the relay that switches the outdoor unit on, or the capacitor that helps it start. An outside AC unit not running with a buzzing sound at the contactor points to those two parts.

AC Compressor Not Turning On

When the outdoor fan spins but the air stays warm, the compressor is not running. You can hear the difference: a running compressor adds a deep steady hum under the fan noise. An AC compressor not turning on is most often a failed capacitor, because most units use one dual capacitor to start both the fan and the compressor, and half of it can fail alone.

An AC compressor not kicking on at all is a different sign from one that tries. A compressor that tries to start, hums for a second and clicks off is tripping its own overload. That can be the capacitor again, low voltage, or a compressor near the end of its life. A new compressor runs $1,000 to $2,500, which on an older system is usually the point to price a replacement with the central air installation cost calculator before agreeing to the repair.

AC Fan Not Spinning

An AC fan not turning on, an air conditioner fan not spinning and an AC fan that stopped working overnight sound like one fault. But an AC fan not working can mean either of two fans, and which one has stopped changes the diagnosis completely. The outdoor fan sits on top of the condenser. The indoor fan, called the blower, pushes air through the vents.

Outside AC Unit Fan Not Spinning

Call it an outside AC fan not spinning, an outdoor AC fan not spinning or the fan on the AC unit not spinning: an outdoor AC unit fan not spinning is the urgent one, so switch the system off at the thermostat as soon as you notice it. With the outdoor fan still and the compressor running, heat has nowhere to go, pressure climbs and the compressor overheats within minutes.

• It hums but does not turn. The classic sign of a failed capacitor. Owners often report that the fan starts when pushed with a stick, which confirms it, but it is not a fix: the capacitor still needs replacing.
• It is silent and so is everything else outside. No power or a failed contactor. Go back to the breaker and disconnect checks above.
• It turns slowly, or starts and stops. A fan motor with worn bearings or a weak capacitor.
• Something is in the way. A stick or debris through the top grille. Cut the power at the disconnect before reaching in.
• The test owners use. With the power off at the disconnect, turn the blade by hand. Owners and technicians repeat the same rule: a blade that spins freely points to the capacitor, and one that is stiff or gritty points to the fan motor.

Two owner reports are worth knowing before you buy a part. A capacitor can look perfect and still be dead, because the bulged top people look for is a sign and not a test. And owners who changed both the capacitor and the contactor and still had a humming unit found the fan motor was the fault. A new capacitor is the cheap outcome, and it is the usual one. A capacitor holds a charge even with the power off, so this is a part to have fitted unless you know how to discharge it safely. The AC repair cost calculator gives the fitted price for your region.

AC Running but Not Blowing Air From the Vents

If the outdoor unit runs but little or no air comes out of the vents, the blower is the place to look. An AC not blowing air, or an air conditioner not blowing air from some vents only, has four usual causes.

• A filter packed solid. Air cannot get through it. Replace it before anything else.
• A coil buried in ice. Ice blocks the air path completely. See the section on thawing a frozen AC.
• A failed blower motor or its capacitor. The outdoor unit runs, the indoor unit is silent. A blower motor is a $300 to $800 repair.
• One room only. A closed damper, a crushed flex duct or a duct that has come off its boot. That is a duct problem, not an AC problem.

Tip: never leave the outdoor unit running when the indoor fan is not moving air. With no air over the coil it freezes within the hour, and then you have two faults to pay for.

AC Keeps Tripping Breaker: What the Timing Tells You

A breaker is a measuring device, so an AC tripping breaker is a reading, not the fault. An AC unit tripping breaker is telling you the current went past the limit, and when the AC keeps tripping the breaker, or the air conditioner breaker keeps tripping day after day, it is saying so again. What separates a cheap cause from an expensive one is the timing.

• It trips the instant the unit starts. An AC tripping breaker immediately, or an AC compressor tripping breaker, is the serious version. Something is drawing a huge current at once: a shorted wire, a capacitor that has failed so the compressor cannot turn over, or a compressor winding shorted to its case. Leave the breaker off.
• It trips after ten minutes or an hour. The system is working harder than it should and the current creeps up as it heats. A dirty outdoor coil and a clogged filter are the usual reasons, and a fan motor near the end of its life is the next.
• It trips only on the hottest afternoons. Same cause, closer to the edge. Hose the outdoor coil clean with the power off before anything else.
• It is a window or portable unit. The usual story behind window AC tripping breaker complaints is a unit sharing a household circuit with something else. Move the other load, or give the unit an outlet of its own, before blaming the AC.
• The breaker itself feels warm or loose. Breakers wear out, and a loose connection at the panel heats up and trips a healthy circuit. That is an electrician's job, not an HVAC one.

If you want to know what your unit should draw before a technician arrives, the central AC amps by ton table gives the running amps for each size. A breaker that keeps tripping is not dangerous while it stays off. The danger is in holding it on or resetting it again and again, because each reset sends the same fault current through the wiring. If the AC breaker tripped, reset it once. If it trips again, the breaker is not the problem, and fitting a bigger one only moves the failure into the wiring.

AC Short Cycling: Turning On and Off Every Few Minutes

Short cycling means the unit starts, runs briefly, stops and starts again without ever finishing the job. You may know it as AC short cycling, an AC unit short cycling, an AC unit turning on and off, or air conditioner short cycling on mild days only. An air conditioner turning on and off like this is being stopped by something, and the list is short.

When the AC keeps shutting off, the AC keeps turning off after a few minutes or the AC keeps turning on and off all afternoon, it is one fault. One pattern is different: if the AC turns on then off immediately, within seconds, the compressor is failing to start, and that belongs with the breaker section above.

• Low airflow. A clogged filter or an iced coil trips a safety switch, the unit rests, and it tries again. Start here, because it is free.
• A dirty outdoor coil. Heat cannot leave, pressure climbs, the high pressure switch opens. It resets when things cool down, and the cycle repeats.
• Low refrigerant. The low pressure switch does the same from the other side. It gets worse over weeks, not overnight.
• The thermostat is in the wrong place. Above a supply vent, in direct sun or beside a kitchen, it reads its own patch of air and not the house.
• A full drain pan. A float switch that trips, drains a little and resets gives an AC unit that turns on and off all day.
• The unit is too big. An oversized system cools the air near the thermostat in minutes and shuts off before the rest of the house catches up.

Short cycling is worth fixing even when the house stays cool. Starting is the hardest moment in a compressor's day, so a unit that starts ten times an hour wears faster, uses more power and removes less moisture than one that runs steadily. If it is cycling every minute or two, switch it off until you have checked the filter and the coil.

The last cause on the list is the only one you cannot repair, and it has a signature: it has cycled this way since the day it was installed, and the house feels cold and clammy. Check the size with the central AC size calculator before spending anything on parts. Tip: short cycling that started recently is a fault, and short cycling that has always been there is the size.

AC Making Loud Noise: Buzzing, Humming, Rattling and What Each Means

An AC making loud noise is telling you which part is in trouble. An AC unit making buzzing noise at start-up points to the capacitor, an AC making humming noise with nothing moving to a motor that is not starting, and a noisy AC unit that was quiet last summer to a part wearing out. With an AC making noise, the sound is the clue, whether it is an AC making buzzing noise or an AC humming. A noisy air conditioner is easier to diagnose than a silent one, because each sound comes from a different part. Work out whether the indoor or the outdoor unit is making it, then match it below.

When the sound comes matters as much as what it is. When an AC makes loud noise when turning on, it is usually a compressor struggling to start: loud for a second, then quiet, which points back to the capacitor. A single clunk as it shuts down is the compressor stopping and is normal. A bang from the ceiling a few seconds after start or stop is sheet metal ductwork flexing as the pressure changes, which is a duct matter and not a fault in the machine. An outside AC unit making loud noise all the time is the one to act on, whether it is the AC compressor making loud noise or the AC fan making noise, and so is an AC making a rattling noise that gets worse week by week.

An AC unit making a loud noise gets attention. An AC unit making a buzzing noise is the one people put off, because the house is still cool. It is also one of the cheaper repairs: replacing a capacitor or contactor runs $150 to $450, and replacing it early protects the motor it is failing to start. Whether it is safe to keep running depends on the sound: banging, grinding and a scream from the compressor mean off now, while a rattle or a brief hiss can wait for daylight. Tip: a new noise with the cooling still working is the cheap moment to call. The same noise a month later usually arrives with a second fault.

How to Reset AC Unit Power, and Where the Reset Button Is

The steps below show how to reset AC unit controls and how to reset air conditioner breakers. As for where the AC reset button is, on most central systems there is nothing to press. People look for a reset button because other appliances have one. Here is how to reset an air conditioner with the switches it does have.

1. Set the thermostat to off. Not to a higher temperature, off.
2. Switch off both AC breakers. One for the indoor unit and one for the outdoor unit.
3. Wait a few minutes. That lets the control board lose its charge and the pressures in the system settle.
4. Switch the breakers back on, then set the thermostat to cool. Give it five more minutes before you judge it, because the compressor is held off on purpose after any power cut.

A reset button on outside AC unit cabinets is rare: on most there is none, and the gray box on the wall beside it is a disconnect, which only cuts the power. Knowing how to reset AC circuit breaker switches is the part that matters, and it means fully off, then on. There is no AC compressor reset button on a typical home system either, since the compressor resets itself once it cools. Where a real AC reset button exists, it is one of two things. Some outdoor units have a small red button near the copper lines, which is a high pressure cutout, and a unit that needs it pressed more than once has a fault that the button will not fix. Window and portable units carry TEST and RESET buttons on the plug, and a plug that keeps tripping is protecting you from a damaged cord.

A reset clears a control that has locked itself out, and nothing else. If the AC runs normally afterwards, a power dip was probably the cause. If it stops again within a day, the lockout was the system protecting itself, and the cause is elsewhere on this page.

When It Is Sizing, Not a Fault

Some complaints that look like faults are the system doing exactly what it was installed to do. A unit that cools quickly then leaves the room cold and clammy is usually oversized rather than broken, because it satisfies the thermostat before it has removed much moisture. One that runs constantly on the hottest days without ever reaching setpoint may simply be too small. Neither is diagnosed by opening anything up. If the behavior has been there since installation rather than appearing suddenly, check the sizing with the AC tonnage calculator or, for one room, the window AC size calculator before spending anything on repairs.