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5 Potential Causes Of Your A/C Problems When The Outdoor Unit Refuses To Start

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While there are dozens of problems that can interrupt your enjoyment of cool air from the air conditioner, only a few of them completely stop the outdoor unit from turning off. When you notice there's no noise at all coming from the unit, it's time to start troubleshooting the problem. Here are five potential causes, some of which you can check yourself before calling for repairs.

Lack of Electricity

Start by making sure the unit is plugged into a 220 outlet or that the permanent wiring isn't disconnected. The unit must have a source of power in order to switch on, and it's easy for kids or pets to accidentally unplug the cord or damage the exterior wiring. If that's not the problem, there may be disconnected wires inside the unit, but it's best to leave interior work to the professionals because of the high voltage left in the capacitor and other parts. Wires chewed by pests and damaged resistance contacts also cause a lack of electrical power to the exterior unit, but they require attention from an air conditioning repair professional.

Tripped Switch

Don't forget to check the circuit box to see if the breaker for the A/C has been tripped by a power surge. It's a shame to call out a repair technician and pay for the visit when it's simply a matter of flipping a switch inside your house. Check the fuses in the box as well to make sure it's not a simple matter of switching out a single inexpensive electrical part. Other than the breaker, other potentially tripped switches include

  • The emergency disconnect switch located where the unit draws power from the house
  • The secondary emergency shutoff switch on the inside of the unit, which should be labeled on the exterior so you don't have to dig through the interior
  • The reset button located on the exterior of the unit, which is used when the A/C stops unexpectedly.

Sensor and Module Damage

The outdoor condenser unit is full of sensors and modules that control the flow of power to different components. For example, there are sensors that turn off the unit if high heat, unusually high or low pressure, or electrical shorts are detected. These sensors can also break and act up when the A/C is in fine working shape, so only a professional can help you track down exactly which module or sensor is keeping your unit from turning on.

Dead Motor

Do you hear the gentle hum of the compressor when you try to switch on the A/C but no louder whirring from the unit and no air in the house? This means your compressor is working but the fan motors are not working. This could be due to a simple loose wire, but it's more likely a sign that you need a new motor. This is an issue that needs diagnosis from a technician with a ohmmeter so they can determine if it's just a lack of power or a genuinely burnt out part because replacement motors can cost hundreds of dollars.

You can try opening up the case and seeing if you can spin the blower fan blades by hand. If the blades are stuck or hard to spin, you've definitely got a bad motor.

Clogged Vents

Sometimes A/C units that only turn on occasionally are more frustrating that outdoor units than simply sit there and react to nothing. Does your condensing unit turn on after an hour of two of waiting but then shut off after a few minutes and refuse to start up again? This is a sign you need to clean out the vents and remove debris from the inside of the case. A lack of air flow results in overheating, which trips the emergency sensor. The wait allows it to cool off enough to restart.


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