The condensing coils are located on your outdoor AC unit, which is usually installed next to the home. They generally look like thin vent openings, or fins, along one or more sides of the unit. Coils can be fully exposed or they may have a metal grid over them to provide some protection. The coils handle the cooling of the hot air with the help of the refrigerant in the unit, so if they fail, your unit will no longer cool your home properly.
The most common problem with the condensing coils is dirt. Dust, mud, pollen, and plant debris can lead to dirt buildup on the outside surface of the coils. If you have a dog that lays down near the unit, then pet hair can also be a problem. Mold is a common issue on the inside surfaces of the coils, especially in humid or damp climates. Hosing the coils off periodically will remove surface dirt. Do this on warm days so that the excess moisture dries quickly and doesn't lead to mold. For severe dirt buildup or dirt inside the unit, your AC repair tech can clean the coils during a service visit.
In order to properly cool, the fins need to be straight. Even minor bends can reduce the cooling capacity of your AC unit. Many things can bend the delicate coils — wind damage, plants growing too close to the unit, a wayward ball from a playing child, etc. You can minimize the chances of damages by putting a screen up, such as a fence, to protect the AC from the rest of the yard. Just make sure to set the screen at least 2 feet from the unit so air circulation isn't affected. If the fins are bent, then a repair tech can straighten them with a special tool called a fin comb. If there is severe damage, the condenser coil may require replacement.
Condenser coils are made of rust and corrosion-resistant metal, but they aren't immune to damage. The presence of chlorine and other chemicals in the air can increase corrosion chances, as can basic age, weathering, and cleanliness. In the early stages of corrosion, a service tech may be able to sand off the damage. If the damage is extensive, you will need to have the condenser coils replaced.
Contact an AC repair service in your area if you need more help with your condensing coil repair or service.