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.
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If your AC is not working at all:
1. Be sure it is receiving power at the electrical panel.
2. Reset the equipment’s switches and/or overloads.
3. Check the thermostat settings.
4. Check the condensate overflow switch.
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.
5. Check and clean the outdoor compressor.
Several different kinds of problems can occur with a central air-conditioning system. These are some of the most common ones that we discuss below:
• AC not working at all
• AC not cooling well but runs
• Room temperature too hot or too cold
• Air conditioner drips or leaks water
• Air conditioner makes strange noises
• Air conditioner won’t turn off
Although some central air conditioner repairs must be handled by a qualified air-conditioning repair person, in this article we’ll look at how you can handle simple repairs and maintenance yourself.
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. You should replace the filters at least twice a year, before the heating and cooling seasons.
If your central air conditioner does not 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 should be on a dedicated 240-volt circuit.
If the breaker continues to trip, there is probably a short in the system—in the compressor, capacitor, or the fan motor. You’ll need 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 be sure the compressor’s 240-volt disconnect, typically in a metal box mounted near the compressor, hasn’t been shut off.
4Remove the thermostat’s cover after turning off the power to the air conditioner. Remove the thermostat’s body from the base (usually by pulling straight out) and replace the batteries (if it has batteries). Be sure all wires are properly attached to their terminals and will not be pinched by the cover. Replace the cover and wait 3 or 4 minutes, and then try the system again.
5If that doesn’t 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 doesn’t 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.
6Check the compressor’s 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. It’s very easy to test whether it works and it is cheap and easy to fix if it doesn’t.
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. 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 number—not “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.
How to Test an Air Conditioning Condensing Unit Capacitor and Wires from oliverfails on Vimeo.
Still not working? Please read “Air Conditioner Not Blowing Air,” below.
If you can hear your central air conditioner running, but it doesn’t cool well, the problem can be caused by a couple of issues. Make sure nothing is blocking or limiting air flow anywhere in the system—the 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 isn’t 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.
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 is something that must be checked and recharged by a pro. Reduced air flow is the most likely problem.
4Clear 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 won’t work—in fact some condensate drains are equipped with a float switch that won’t let the AC run if water backs-up. Water is also likely to puddle around the unit or even 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, be sure to shut off all power to the unit and verify that it is off.
2Clean the outdoor compressor. For complete instructions, please see Air Conditioner Doesn’t 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 isn’t 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 Have the coolant checked. If none of these steps worked, the chances are pretty good that the coolant needs to be recharged by an air conditioning professional. Call a qualified local AC professional—this is not something you can do yourself.
If your room has dramatic swings in temperature before the system kicks on, see Room Temperature Too Hot, Then Too Cold.
If your AC doesn’t go on at all when you set the thermostat for a cooler-than-room temperature, please read Central AC Not Working At All, above. Then, if you have fairly good DIY skills and a multimeter, go through the diagnostic steps in this video to determine whether the problem may be a condensate pump switch.
If your air handler isn’t blowing air properly, the indoor evaporator coil may be dirty or frozen up. Reduced air flow, whether caused by dirty filters, a dirty coil, dirty squirrel cage blower, or a broken fan motor will cause the coil to freeze up, blocking air flow.
If your air handler’s motor runs but the blower doesn’t move air, the belt that connects the two probably has broken. Replacing it is an easy fix if you have a few tools and do-it-yourself skills.
Here’s how to replace the air conditioner’s blower belt:
1Turn off all power to the unit and, 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 to give you access to the blower (it might be on a slide-out drawer.) Check the number stamped on the belt and get an exact replacement from a home center or heating supply outlet.
3You can usually slip the belt on the motor’s (smaller) pulley first and then start it on the blower pulley, as shown at right. Rotate the blower pulley by hand, holding the belt in place but keeping your fingers from getting caught between the belt and the pulley. The belt should slip right into place. If it seems to be too tight or difficult to set in place, it may be necessary to adjust the motor mount to provide more slack. Then you can re-tighten the tension once the belt is in place. Check the manufacturer’s specifications for proper tension—in most cases, the belt should deflect about an inch when you press down on it.
4Lubricate the blower. Finally, some blower motors need oiling; some have sealed bearings. If recommended by your maintenance manual, oil the bearings according to the manufacturer’s directions.
If the blower motor hums but doesn’t run, you’ll probably need to replace the blower motor. This video shows how to replace the motor.
If the air handler doesn’t run at all, it may not be receiving power. Check its circuit breaker, and any switch that controls it at the air handler or furnace cabinet. Then check the door switch—a small switch inside the metal door that prevents the appliance from running when the cabinet is open. Also check the thermostat. For more, see Central AC Not Working At All, above.
Air conditioners and high-efficiency combustion furnaces create significant condensation—we’re talking about a lot of water, especially in humid climates—several gallons per day. This water exits the air handler through plastic pipe or a drain tube. That drain tube goes directly outside, often terminating near the compressor, or to a floor drain or to a small electric “condensate pump” located by the air handler. Where a condensate pump is used, it connects to a 1/2-inch vinyl or rubber tube that exits outdoors or to a drain.
If water is dripping or pooling at the base of the air handler, it may be leaking from one of the plastic pipes or tubes that carry it, something may be blocking the water’s flow, or the condensate pump may not be working. If the pump stops working, it will overflow.
On some air handlers, the condensation drainpipe has a small safety float switch (see photo) connected to it. If the drainpipe backs up with water, the float switch will shut off the air conditioner. This is a sure sign that it’s time to clear the condensation drainpipe.
1 If your system includes an electric condensate pump, make sure it is connected to a working electrical outlet. Then look to see if the tubing that carries away the water has come loose from the pump. If it has, reconnect it.
More likely is the possibility that the tube or the pump is clogged with algae. If this is the case, use a wet-dry vacuum to suck all of the water out of the tube. In some cases, it might be easier to replace it with new 1/2-inch tubing purchased online, at an HVAC supply shop, or at a well-stocked home improvement center.
2 Test the pump by pouring water into its collector. The pump, which is turned on by a ball float inside that rises with the water level, should start. If it doesn’t, it is stuck or broken. If it’s stuck, you should be able to disconnect it from the power and from the inbound tubes, disassemble the top, and clean it out thoroughly. If it’s quite old or broken, you’re probably better off replacing the condensate pump so you don’t have to do this again in the near future.
To kill algae, pour a dilute solution of bleach (1 part bleach to 16 parts water) into the pump’s trough and pump it out through the tube. Don’t do this on you lawn—it will kill the grass.
3 If the pump runs but doesn’t empty the trough, the ball-like check valve just before the discharge tube is probably stuck. Unscrew the check valve, loosen the ball inside, and look for an obstruction and a build-up of algae. If it appears that a condensation drain tube is clogged with algae, remove it if possible (you may have to cut it and replace it later with a coupling). Blow it out or run a wire through it to clear it—or, better still, replace it with new tubing.
4 Ice may be blocking the tube. If this is the case, clean or change your AC filters. If the filters appear to be fine, the air conditioner’s refrigerant supply is probably low. Call an air-conditioning technician to have the unit recharged.
For more about filters, see How to Replace Furnace & AC Filters.
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 Doesn’t Blow Air and refer to your owner’s manual.
If a direct-drive blower is squealing or making a grinding noise, shut off the unit and—unless you’re an accomplished do-it-yourselfer—call an HVAC professional—the motor’s 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 can’t, the run capacitor may be defective. If that capacitor goes bad, the motor will overheat when it tries to turn on but can’t 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 unit—something 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, it’s 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.
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 that’s 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 aren’t, tighten them.
If your AC doesn’t 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 AC’s circuit breaker to shut off the unit, but that should not be a permanent solution—circuit breakers are not designed to be used like light switches.
If the thermostat is blank—not displaying any values—or 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 isn’t 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 correctly—with 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 doesn’t turn off, it’s likely that the contacts on the outdoor run relay have welded together—something 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. Disassemble the relay and pry apart the contacts. This should do the trick until you buy a replacement—which you should do soon.
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Getting the Work Done
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