Troubleshooting A Broken Central Air Conditioner

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Summer is almost here which means it won’t be long before you rely on your central air conditioner to do battle with the oppressive heat that seems to last for weeks at a time. If you haven’t done so already, you should make sure the outside condensing unit is clear of debris and you should replace the air filter in the furnace or air handler in preparation for using the central air system on a daily basis.

If you’re feeling more ambitious, Repair Clinic has ten tips for getting your central air conditioner system ready for summer that you should review today. Number six on that list is to turn on the central air system to make sure everything is in working order. So, what do you do if the central air condensing unit won’t run? Repair Clinic has some answers for that as well; in this article, we’ll show you how to troubleshoot a broken central air conditioner. The first step is understanding how that outside condensing unit actually works.

Understanding how a central air conditioner condensing unit works

Central air cooling requires two separate units to get the job done. The first is a furnace or air handler which is located in the basement, attic, or closet space of the home. The second is a condensing unit, usually mounted on a slab of concrete directly behind the home or at the side of the house or apartment complex. The condensing unit has a series of condenser coils around the outside of the unit with a fan motor and fan blades mounted to a grille on top. Other components found in the condensing unit include a compressor, a filter-drier, a contactor, and a capacitor.

How the furnace or air handler works with the condensing unit

The temperature in the home is regulated by a wall thermostat. When the thermostat is lowered or it detects an increase in temperature, it closes a cooling circuit, allowing voltage to travel to a control board. The control board will then send 120 volts of alternating current to a circulation blower fan in the furnace or air handler and 24 volts to the contactor in the outside condensing unit. When the contactor is energized, it allows 240 volts to flow through a nearby disconnect box to the compressor and condenser fan motor located in the condensing unit.

Dual run capacitors power both the compressor and the condensing unit fan motor

The capacitor, which stores an electrical charge then releases it, may be a single run capacitor or a dual run capacitor. A single run capacitor will power just the compressor or the condenser fan motor, but the majority of central air condensing units will use a dual run capacitor which can power both the compressor and the fan motor.

The compressor pumps refrigerant and the condenser fan helps dissipate heat

The compressor acts as a pump, compressing refrigerant in gas form into the condensing unit’s condenser coils. A filter-drier is often attached to the refrigerant line to remove moisture from the refrigerant. Once the refrigerant gas enters the condenser coils, it is condensed into a hot liquid. The condenser coils dissipate the heat as the liquid travels through them, assisted by the spinning condenser fan motor blades. After passing through the condenser coils, the refrigerant travels to the evaporator coils mounted on the furnace or air handler. There the refrigerant expands back into a gas which makes the coils cold. The circulation blower fan draws air past the cold evaporator coils and forces the air through the venting to cool the interior of the home.

Central air condensing unit won’t run? 3 likely causes

When troubleshooting a broken central air condensing unit that won’t run, there are three likely causes you should investigate:

  1. Faulty contactor – As we noted earlier, when the central air conditioner contactor receives a signal from the furnace or air handler control board, it allows voltage to be sent to the condensing unit components. A faulty contactor will not allow voltage to be sent to condensing unit, preventing it from running. You should visually inspect the contactor contacts for signs of wear (there will likely be burn marks near the terminals if the contactor has failed). You can also test the contactor with a multimeter. Use the multimeter leads to touch in the incoming contacts to confirm the contactor is receiving 240 volts. After setting the wall thermostat to call for cooling, test the contactor coil terminals to determine that 24 volts are present. If 24 volts are present, it’s likely the coil or the outgoing contacts are damaged and the contactor will need to be replaced.
  2. Malfunctioning thermostat – If the contactor coil terminals do not show that 24 volts are present, it’s likely the wall thermostat is malfunctioning. Set the thermostat for cooling and wait for the contacts to close. Next, detach the yellow wire from the thermostat terminal and touch the multimeter leads to the red (“R”) and yellow (“Y”) terminals to check for electrical continuity – a continuous electrical path present between the two terminals. If there is no electrical continuity, you’ll know the wall thermostat is malfunctioning and you will need to install a new thermostat to fix the problem.
  3. Incoming Power Problem – Now let’s go back a couple of steps. If the incoming contactor contacts did not show that 240 volts were present when you were testing the contactor, there is an incoming power problem. Open up the condensing unit’s disconnect box (usually found mounted to the side of an exterior wall directly behind the condensing unit) and check to see if a circuit breaker inside the box has tripped. If so, switch the breaker back to the “On” position. If the disconnect box uses fuses instead of a circuit breaker, you can remove the fuses and use the multimeter to test each fuse for electrical continuity to help determine if the fuse has blown. A fuse that tests negative for continuity should be replaced with a new one.
Central Air Conditioner Condensing Unit Not Running?

Repair Clinic will help you fix your central air condensing unit

Need to install a new contactor in your central air condensing unit, but don’t know how? Check out the “Videos & Articles” section of the Repair Clinic website and enter the full model number of the condensing unit in the search bar. Repair Clinic has model-specific schematics, diagrams, and step-by-step procedural videos such as this one demonstrating how to replace a contactor  on a York (Model TCJD24S41S3A) central air condensing unit, or this video showing how to install a new White Rodgers Sensi Smart Wall Thermostat.

Use only genuine OEM parts when fixing a central air condensing unit

As your repair partner, Repair Clinic has your back when it comes to quality replacement parts. We stock only genuine OEM (original equipment manufacturer) parts from such name heating and cooling brands as Lennox, Frigidaire, Goodman, Rheem, York, Carrier, Payne, and Honeywell, so you can be assured that your central air condensing unit will be fixed right the first time. Enter the full model number of the condensing unit in the Repair Clinic website search bar to see a comprehensive list of genuine manufacturer parts compatible with the unit.

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