AC Not Working | Central Air Conditioning Repair & Troubleshooting
Don Vandervort, HomeTips
Clean the central air conditioning compressor, located outdoors, before every cooling season.
Is your AC not working? If your central air conditioner is not cooling right, save money, time, and get comfortable again with these expert DIY air conditioning troubleshooting and repair techniques.
If your AC is not working at all:
1. Be sure it is receiving power at the electrical panel.
2. Reset the equipments switches and/or overloads.
3. Check the thermostat settings.
4. Check the condensate overflow switch (see below).
If your AC is not cooling but runs:
1. Turn off the power.
2. Clean or change the filter .
3. Look to see if ice has formed on the coils and, if it has, turn on the power and the fan to melt it.
4. Clean the condensate drain (see below).
5. Finally, check and clean the outdoor compressor.
Now, In-Depth Answers to Your AC Problems:
Several different kinds of problems can occur with a central air-conditioning system. We discuss some of the most common ones below:
AC not working at all
AC not cooling well but runs
Room temperature too hot or too cold
Water leaks from air conditioner
Air conditioner makes strange noises
Air conditioner wont turn off
Although a qualified air-conditioning repair person should handle certain types of central air conditioner repairs, youll find methods for handling simple repairs and maintenance yourself in this article.
In addition to the information below, see these two articles for the general care and maintenance of your air conditioner: Preparing Your Air Conditioner for Summer and How to Replace Furnace & AC Filters . Most noteworthy, you should replace the filters at least twice a year, before the heating and cooling seasons. For information on furnace problems, please see Furnace Not Working .
Central AC Not Working At All
Mariusz Blach | Dreamstime.com
First make sure the thermostat is set to Cool if your AC wont go on. Shop for programmable thermostats now .
If your central air conditioner wont go on automatically when the thermostat signals the need for cooling:
1Check the main electrical panel and any secondary circuit panels for a tripped breaker or blown fuse . If you find the problem there, reset the breaker by turning it off and then on or replace the fuse. A central air conditioner typically connects to a dedicated 240-volt circuit.
If the breaker continues to trip, suspect a short in the systemin the compressor, capacitor, or the fan motor. Hire an electrician to track down and solve this problem.
2Make sure the thermostat is set to COOL and its temperature setting is at least 3 degrees below the ambient room temperature.
3Make sure the power is on. Check the switch in the furnace or air handler, and the outdoor condenser. Also make sure no one has shut off the compressors 240-volt disconnect, typically in a metal box mounted near the compressor.
With this type of electronic thermostat, you pull off the body to access the baseplate and wires.
4Remove the thermostats cover after turning off the power to the air conditioner. Remove the thermostats body from the base (usually by pulling straight out) and replace the batteries (if it has batteries). Make sure all wires securely attach to their terminals and that the cover wont pinch them. Replace the cover and wait 3 or 4 minutes, and then try the system again.
5If that doesnt do the job, open the thermostat and unscrew the wire from the Y terminal. Turn the power back on. Holding the wire by its insulation only, touch the bare end to the R terminal and hold it there for about two minutes. If the compressor kicks on, the thermostat is faulty; replace it as discussed in the article How to Install an Electronic Thermostat . If the compressor doesnt go on when you hold the Y wire to the R terminal, turn the power back off and either call an air-conditioning technician or check the capacitor.
6Finally, check the compressors capacitor and wires. The capacitor in the compressor starts both the condenser and the fan. If the capacitor has failed, the A/C unit will not run. Its very easy to test whether it works and it is cheap and easy to fix if it doesnt.
How to Test the AC Compressors Capacitor
Before opening the electrical cover on the A/C unit, be sure to shut off all power to the unit and verify that it is off. Next, remove the cover and, using a digital multi meter set to Capacitance, put one lead on the Common terminal and the other lead on one of the other two terminals. The meter should show a numbernot OL which indicates a short. A/C capacitors can be like two capacitors in one, with both sharing the common leg.
Here is a video that shows more precisely how to do this. Tip: Take a quick digital photo of the wires before disconnecting them so you know where to replace them.
Still not working? Please read Air Conditioner Not Blowing Air, below.
AC Not Cooling Well
If you can hear your central air conditioner running, but it doesnt cool well, make sure nothing is blocking or limiting air flow anywhere in the systemthe air filters, registers, and the compressor. For more about filters, see How to Replace Furnace & AC Filters .
A kinked refrigerant line, or a blower fan that isnt running right can cause this, too. Next, check the air handler:
How to Check the Indoor Air Handler
1Turn off the power to the air conditioning air handler or furnace. If the air handler is a gas furnace, turn off the gas at the gas valve that serves the furnace.
2Remove the door on the front of the air-handler cabinet so you can access the filter. Pull out the filter and clean or replace it as necessary.
3Look for ice. If you see ice in the area around the coils, close the unit back up, turn the power back on, and turn on the fan. The ice should melt within an hour or two.
Ice In the AC Air Handler
Only two things cause an AC air handler to freeze up: 1) reduced air flow because of dirty filters, coils, or poorly working fans, and 2) low refrigerant level, which a pro must check and, if necessary, recharge. More often than not, the problem comes from reduced air flow.
Clear the Condensate Drain
Air conditioners can create a lot of water because they remove moisture from the air. To get rid of this, they have a [usually plastic] drain pipe that comes out of the side of the air handler. Over time, algae can block this pipe and, when it does, the AC wont work. In fact, some condensate drains have a float switch that wont let the AC run if water backs-up. Water can also puddle around the unit or flood the area. To deal with condensate problems, please see Air Conditioner Leaks Water, below.
How to Check the Outdoor Compressor
1Turn off the power. Before cleaning the compressor, shut off all power to the unit and verify that it is off.
2Clean the outdoor compressor. For complete instructions, please see Air Conditioner Doesnt Cool Well . This video does a good job of showing you how to clean the coils:
3Make sure the compressor is working. When you set the thermostat to a temperature below the room temperature, you should hear the outdoor compressor run and see the fan turning inside the top. If the fan isnt turning, look for an overload button or switch to reset (not all types have this). Stick a screwdriver or stick down through the top grille and try to spin the fan blades clockwise. They should spin freely. If doing this gives the fan enough of a boost to get it going, the unit has a faulty capacitor that must be replaced. See Step 6 under Central AC Not Working At All, above.
4 Finally, have the coolant checked. If none of these steps worked, the coolant may need recharging by an air conditioning professional. Call a qualified local AC professional this is not something you can do yourself.
For more about filters, see How to Replace Furnace & AC Filters .
Air Conditioner Makes Noise
Noises sometime occur in the indoor air handler, the outdoor compressor, or the ductwork.
Air Handler Makes Noise
Problem noises in an air handler are squealing, grinding, and buzzing.
AC air handler squealing or grinding noises. Though most air handlers have direct-drive motors, some older units may be belt-driven. Squealing sounds from a belt-drive air handler generally occur when the belt that connects the motor to the blower slips. In most cases, the belt is improperly aligned or worn and needs replacement. Follow the instructions above under Air Conditioner Doesnt Blow Air and refer to your owners manual.
If a direct-drive blower is squealing or making a grinding noise, shut off the unit andunless youre an accomplished do-it-yourselfercall an HVAC professional the motors bearings are probably shot. If you can handle this type of work, you can order and replace the motor.
AC air handler buzzing sounds. Do you hear a buzzing sound when you turn on the thermostat? The only thing that comes on in the house when you turn on the thermostat is the fan relay and fan in the air handler or furnace. Try switching the thermostat HEAT/COOL switch to OFF. Then switch the fan switch from AUTO to ON. The fan (only) should come on. If the air handler makes a buzzing sound, it probably has a bad fan relay or, more likely, blower fan.
Outdoor AC Compressor Makes Noise
Outdoor compressors sometimes make buzzing, humming, or grinding noises.
AC compressor buzzes. If the outdoor AC unit makes a buzzing sound, as though it wants to turn on but cant, the run capacitor may be defective. If that capacitor goes bad, the motor will overheat when it tries to turn on but cant get started. When the motor overheats, a thermal overload switch will shut it off.
When it buzzes, quickly poke through the grille with a long screwdriver or stick to try to spin the fan clockwise (be careful!). If it starts spinning, it will run for one cycle but then stop working again. This indicates that the capacitor is bad and needs to be replaced. (See Central AC Not Working At All, above.)
AC compressor hums. If the outdoor compressor hums when you switch the thermostat to COOL, the problem could be the low voltage transformer on that unitsomething that would need to be checked by a pro.
AC compressor grinding noise. Over time, the bearings in the motor can wear out. The result is a grinding sound and, in many cases, the motor will overheat and shut off. When this happens, its time to replace the motor.
Many heating/cooling ducts are metal so they conduct noise quite readily from the air-handling unit to your rooms. To break the conduction of sound, you can have a heating contractor insert flexible insulation ductwork between the heating/cooling system and the metal ductwork.
Ernest R. Prim / Shutterstock.com
Hire a pro for complex repairs. Here, an HVAC professional checks the refrigerant levels in an air conditioning system. Get an HVAC pro now .
Ductwork pinging or popping. If you hear a pinging or popping sound coming from metal ductwork, this may be caused by thermal expansion or by air blowing past a loose flap of metal. Track along the duct runs, listening for the sound. If you find it, make a small dent in the sheet metal to provide a more rigid surface thats less likely to move as it heats and cools.
Furnace or air handler rattling ductwork. If the furnace makes rattling noises when it runs, be sure the cover panels are screwed on tight. If they arent, tighten them.
Air Conditioner Wont Turn Off
If your AC doesnt turn off when room temperature reaches the temperature set on the thermostat, the problem is generally occurring either at the thermostat or in the electrical system that runs the outside condensing unit. You can use the ACs circuit breaker to shut off the unit, but that should not be a permanent solutioncircuit breakers are not designed to be used like light switches.
If the thermostat is blanknot displaying any valuesor if the AC shuts off when you switch the thermostat to HEAT, the thermostat is probably broken and needs to be replaced. Please see Home Thermostats Buying Guide .
If the thermostat works fine with the furnace, the issue probably isnt the thermostat unless one of the wires has disconnected or arced. When the AC is running, try removing the yellow low-voltage wire from the thermostat. Then:
If the unit turns off, either the thermostat is broken or it is wired improperly. Make sure it is wired correctlywith no cross-connected wires. If the wiring appears to be okay, buy a new thermostat and install it according to the instructions.
If the AC doesnt turn off, the contacts on the outdoor run relay may have welded togethersomething that can happen over time because of frequent electrical arcing at the relay. Before checking the run relay, turn off the power to the furnace and the condensing unit. Next, disassemble the relay and pry apart the contacts. This should do the trick until you buy a replacementwhich you should do soon.
More Answers Below!
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Getting the Work Done
By keeping your AC unit working well, you can save energy and, as a result, money. If a central air conditioner is in ill repair, it probably wont work when you need it most.
If you would rather hire an air-conditioning pro to inspect and maintain your AC equipment, HomeAdvisors FREE service will help you find a qualified local AC professional .
Call for free estimates from local pros now:
AC Not Working | Central Air Conditioning Repair & Troubleshooting was last modified: November 9th, 2017 by Don Vandervort, HomeTips 1997-2017