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DIY Air Conditioner Repair

Our expert shows you how to repair the most common central air conditioning problems by replacing three parts. You'll be up and running sooner and will probably save the expense of a service call.

By the DIY experts of The Family Handyman Magazine

DIY Air Conditioner Repair

Our expert shows you how to repair the most common central air conditioning problems by replacing three parts. You'll be up and running sooner and will probably save the expense of a service call.

Overview: Central air conditioner failures and solutions

You can't cool off in front of the open fridge forever. It's time to decide: You can either wait four days for the service guy to show up or try fixing your central air conditioner yourself. I'll show you which A/C failures can be handled by a DIYer and how to safely replace the three parts that cause the majority of all outdoor condenser unit failures. You'll need a standard multimeter, an insulated needle-nose pliers and ordinary hand tools.

I'll assume you've checked the A/C and furnace circuit breakers in the main electrical panel, as well as any cartridge fuses in the outside disconnect. Replace all three parts at once (about $150 total; see “Buy the Right Parts,” below). Of course, that might mean you'll replace some good parts. But if the fixes work, your A/C will be up and running much sooner and you'll save about $150. Or you can replace the parts one at a time and test the unit after each one.

If these fixes don't work, at least you've covered the most common failures, and your service guy can concentrate on finding the more elusive problem. Plus, with the new parts, you'll likely add years of breakdown-free air conditioning.

Step 1: Make sure the problem isn't the furnace

Set your thermostat to A/C mode and lower the temperature setting. If the furnace fan kicks in, the problem isn't in the furnace. If the fan doesn't run, try resetting the furnace circuit breaker. If the fan still won't start, call a pro—the fixes shown here won't work.

Next, check the outside condensing unit. The compressor (which sounds like a refrigerator) and fan should be running. If not, follow the troubleshooting and repair procedures shown here.


Turn off the A/C and furnace breakers in the main electrical panel before pulling the outdoor disconnect or removing the condensing unit's access panel. Then use a voltage tester on the wires coming into the contactor to make sure the power is really off.

Step 2: Clean or replace the contactor relay

The contactor relay switches power to the condenser fan and the compressor. It rarely fails. But it often gets jammed with beetles, bugs and spiders that perished checking things out. Remove the condenser unit's access cover and locate the contactor relay—it'll have at least six wires attached to it. Try cleaning the critters out with compressed air (Photo 1). If that works, fine. But if you can't remove all the fried bug parts, replace the contactor with a new unit (about $30). Don't think you can file the contacts to clean them. That fix won't last.

Buy the Right Parts
Buy replacement parts from your local appliance parts store or A/C dealer. You'll need the make, model and serial numbers from the nameplate on your outdoor condensing unit—not the furnace nameplate. Or, if you're willing to pay for overnight delivery, you can buy discount parts online (source1parts.com is one site).

Step 3: Replace the capacitor(s)

The “start” and “run” capacitors store electrical energy to jump-start the compressor and fan motor. You may have a combination start/run capacitor or two individual ones. Both styles have a very high failure rate, and when they go, the compressor or fan won't start. They're cheap (about $30), so replace them.

First discharge any remaining electrical charge from the capacitor(s) before you work on them. Fabricate a shorting resistor pack by twisting four 5.6k-ohm, 1/2-watt resistors in series (part No. 271-1125; about $1.25 at Radio Shack). Then discharge the capacitor (photo 2).

Next, move each wire lead from the old capacitor to the new capacitor (photo 3).

Step 4: Replace the fan motor and restart the A/C

Remove the fasteners that hold the fan guard in place or remove the entire cover assembly from the condenser unit. Lift out the fan assembly and mark the bottom of the blade so you replace it in the right direction. Then loosen the blade-retaining nut and pull it off the motor shaft. Disconnect the fan motor electrical connector. Then swap in the new fan motor (photo 4). Reconnect the fan guard or condenser cover.

Reinstall the access cover and the outside disconnect block. Raise the temperature on the thermostat. Then flip the A/C and furnace breakers to “On.” Wait 15 minutes for the thermostat and furnace electronics to reset. Then lower the temperature setting. The condensing unit should start up. If it doesn't, your system may need more time to reset. Wait one hour and try it again. If it still doesn't work, schedule a service call. Be sure to specify exactly what you did. That'll keep the repair person from replacing brand new parts, and you'll be able to have your work checked by an expert.

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • Flat blade screwdrivers (2)
    • Adjustable wrench
    • Drill/driver, cordless
    • Needle-nose pliers
    • Voltage tester
    • Nut driver

You'll also need a multimeter.

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

    • Compressed air
    • Resistor pack

Comments from DIY Community Members

Share what's on your mind and see what other DIYers are thinking about.

1 - 3 of 3 comments
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July 30, 4:50 PM [GMT -5]

Some manufacturers (including the one that made my central AC) will NOT sell parts such as the main contactor, condenser fan thermostatic switch, or even electrical schematics to the consumer. I can only get the parts via an AC contractor and he may or may not be willing to sell them to me so that I can install the part(s)

February 26, 3:59 PM [GMT -5]

This is the best site! I can always come here if something isn't working properly! Like we need air conditioner repair in Denver and of course you can show me how! Thanks for posting!

December 31, 12:34 AM [GMT -5]

Very informative and useful article. Replacing old capacitors to new is very sensitive issue. Some time old capacitors and other refrigerator appliance parts are not available in the for old AC. Installing fan blades are easy job.

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