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Central Air Troubleshooting

If your air conditioner is not working at all, the first thing you will want to do is check if the unit is getting power. Check the circuit breaker or fuse box to see if it has been tripped. If it has, turn it off then on again to reset it. You may also check for blown fuses in the unit. If there are blown fuses, they will need to be replaced. If this does not help, try checking the wires to and from the unit for damage. If there is damage, you will need to repair or replace them.

If the breaker is not tripped and there is no wire damage, you may want to check the thermostat on the unit. Sometimes the thermostat is not set low enough, therefore preventing it from turning on. Another problem that is an easy fix is to check that all switches to and for the unit are switched to “ON.” Usually, there is an external safety switch outside near the condensing unit that you will need to check as well.

You may also need to check the condensate overflow if it is full the switch will not allow the unit to continue running. In this case, empty the overflow and see if you need to repair or replace the condensate pump. If none of this works, there may be a problem with the thermostat, motor or compressor.

Central air unit not blowing air

If there are other problems, it could have caused the evaporator coils to freeze up which will reduce air flow, so check for this problem first. Then check all filters, air registers, outside unit, and ductwork for dirt or blockages. If dirty, clean or replace filters and air registers. If you have damage or a blockage in the ductwork, you will need to remove the blockage and repair or replace the ductwork as needed.

If the air handler motor is running but the blower is not moving air, check to see if the belt that connects the two has broken. If so, you will need to replace this. If the blower motor is humming but not running, there is a problem with the motor and it will need to be replaced. If the air handler is not running, you will need to check for power. If you have power to the unit, then the problem probably lies in the blower motor, capacitor, or the unit’s contactor. You may also try looking for a reset switch on the outdoor unit.

Central air unit not cooling

This can be simple or difficult to fix depending on what is actually wrong with the unit. The first thing you will always want to do is to check the power to the unit and check that the thermostat is set to cool and at a low enough temperature to kick on. Sometimes, depending on the size of the unit and the area it is cooling, the thermostat may need to be set lower than what you would think. Try five-degree increments at first. Then you will always want to check filters, air flow, condenser, and the evaporation coils for dirt, blockage, or freezing.

You should also check the refrigerant in the unit. If there is not enough refrigerant, you will need to call a professional due to regulations and laws. If the compressor and the condenser are both running but the unit is still not cooling, try checking the temperature drop for the unit. This can be done by measuring the temperature of the air coming out of the air handler and subtracting the temperature of the air entering the air handler. Before measuring, the unit should be running for about 15 minutes prior.

For an older unit, the result should be around 18 to 20 degrees and for a high-efficiency unit, the result should be around 15 degrees difference. If the result of the temperature drop is much lower than what it should be, you may have a problem with the refrigerant charge and will need to contact a professional. If the result of the temperature drop is much higher than what it should be, you will need to focus your central air troubleshooting on the air flow system.

The problem could be as simple as it is just too hot outside for your unit to keep up. Depending on how hot it is outside, you may never be able to reach the cold you would like to achieve indoors. This will also lead the unit constantly running and could cause further problems such as the unit freezing up or the motor going bad.

Water leaking from central air unit

Air conditioners and central air units produce a lot of condensation that produces a lot of water. There are a few different ways that this water must exit the air handler. It can be terminated through a plastic pipe or tube that goes directly outside, to a floor drain, or to a condensation pump that will terminate the water outside or to a drain as well. A window unit may leak water inside if it is not sitting in a level position. As for a central air unit, a water leak could be due to drainage issues. Be sure to check the tubing and the drain in which the water exits the air handler to make sure there is no damage or clogging.

If there is water at the base of the air handler, this could be from a leak in the plastic pipes or tubing, a blockage in the water flow, or the condensate pump may not be working which will cause an overflow. If the condensation pump fills and the overflow switch is activated, the unit will shut off until it is drained. You will need to check all the tubing or piping from your air handler and the condensate pump to make sure there is no damage and that nothing has come loose. If so it is easily fixed by repairing the damage or reconnection the pipes. If the pipes have become clogged, you can easily fix this by using a wet-dry vacuum to remove the blockage. Sometimes the pipes can get blocked by ice, which means you need to clean or change your filters.

Ice on the lines or the air handler

If you notice ice on your unit, this means it has frozen up for one reason or another. Usually, ice on the lines or in the air handler tend to be caused by dirty air filters or coils, poorly working fan or low refrigerant levels. After checking the filters and coils, you should check the condensation pump to see if there is overflow, which will cause the unit to freeze up or shut off. Be sure to shut off any power before cleaning the coils on the compressor. Ice on the lines or in the air handler can also restrict air flow, which will only lead to further damage.

You can thaw the ice by turning the outside unit off but let the inside blower run until thawed, which should take a couple of hours. After a couple of hours, turn the unit back on and feel the lines. The larger copper line at the outside unit should be cool and have condensation. If the line is not cold or if frost is forming on it, then you have a Freon problem and will need to contact a professional due to refrigerant regulations. You will also need a professional to check the coolant because it may be low or need to be charged.

Noisy central air unit

Usually, noise will come from the inside air handler, the ductwork, or the outside compressor. Although it is natural for some noise to come from your unit, when you hear grinding, buzzing, or squealing noises, this tends to mean there is a problem.

Newer central air models may not have a belt on the air handler motor, but older ones usually do and this could be the cause of any squealing noises if it has slipped. If this is the case, then you will more than likely need to replace the belt because if will have caused wear on it. If the unit does not have a belt in the motor, then any squealing or grinding noises probably mean that the bearings are bad in which case you need to shut the unit off and call a professional because the motor will need to be replaced.

When you turn on your thermostat, the only thing in the house that turns on is the fan. If you are hearing a buzzing sound, turn the thermostat HEAT/COOL switch to OFF and then turn the fan switch from AUTO to ON. This should turn only the inside fan on. If the buzzing sound is coming from the fan, this could mean that the blower motor is bad or the fan relay is bad.

Of course, the outdoor part of the unit is going to be much noisier than the inside part. There are a few things to look for. If the unit is buzzing and act like it wants to kick on but it does not run, try sticking a screwdriver or stick in through the grill and spinning the fan clockwise, but be careful. If the fan runs, it will run for one cycle and will not run again thus meaning that the capacitor is bad. If the capacitor goes bad, this could cause the motor to overheat causing further damage to the unit.

If you hear a humming noise coming from the compressor, this could be a voltage problem which usually calls for professional attention. If you hear a grinding noise, just as with the indoor unit, it probably means that the gears and bearings in the unit have gone bad and need to be replaced. If not caught early enough, this will also lead to the motor overheating, shutting the unit off and causing more damage.

Ductwork tends to be noisy no matter what, especially if you have metal or tin ductwork. An easy way to help quiet down the noise is to have flexible insulated ductwork installed between the air handler and the metal ductwork. Sometimes you may hear a pinging or popping sound, this can be caused by thermal expansion or the air pushing past a metal flap. You can easily fix this by tracking down where the noise is coming from and then put a dent into the metal sheet. This will cause a more rigid surface that will be less likely to move from temperature. Also, make sure everything is in place and tight.

Central air unit not turning off

Obviously, if your air does not turn off it will be a problem. The first thing you should do is check the thermostat temperature and raise it about five degrees to see if the problem persists. If that does not work, you can temporarily shut it off at the circuit breaker. This should not be done often as breakers are not meant to be used as light switches. If the thermostat is not displaying any values and the air conditioning shuts off when the thermostat is turned to HEAT, then the thermostat is broke and needs to be replaced.

If the thermostat is working fine with the furnace, it probably is not a thermostat problem. There are some things you can do to help narrow down the problem. First, you will want to check all the wiring and make sure nothing has disconnected or arced. Then, while the unit is running, you can try removing the yellow low voltage wire. If the unit shuts off, either there is a wiring issue or the thermostat is broken. Check the wiring and if it still does not work, you can replace the thermostat. If the unit does not shut off when you remove the yellow wire, the contacts on the outdoor run relay may have welded themselves together. You will want to shut off power to the condensing unit and the furnace, then take apart the relay and pry apart the contacts. This should work until you get a replacement.


When it comes to troubleshooting your home central air, poor maintenance is usually the leading cause of the problem. Good maintenance will ensure that your unit is keeping you cool or hot and not eating up more energy than it should lead to higher bills. The first step in maintaining your home central air unit is to keep the filter clean. It is good practice to clean the filters on your central air unit at least once a month. You should also make sure that the air handler coils are clean as well. If you have a window unit, you will want to check the seals around it to be sure no air is leaking in from the outside. You will also need to make sure the air registers are clean and not blocked as this will obstruct the air flow. The compressor on the outside will also need to be free and clear of any plants or anything else that may obstruct the air flow. Be sure not to forget to have your unit serviced annually.  

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