The evaporator coil on your air conditioner handles most of the cooling process. Air moves across the coils, which are filled with refrigerant. If the coils have an issue, then your AC can't properly cool your home.
1. Dirt Buildup
One of the most common issues with the evaporator coils is dirt buildup. A collection of dust and grime interferes with the heat transfer process that the coils are meant to perform on, which means the air blowing across the coils won't cool as sufficiently. Warm air will blow from the vents instead of cool air. Repair is simple, as your tech will use a special coil cleaner to carefully remove any grime that is present.
2. Low Refrigerant
The refrigerant that flows through the coils is the key to cooling the air. If your AC unit has a refrigerant leak, the air won't be able to cool at all. Refrigerant tends to evaporate quickly after it leaks from the unit, so you may not even be aware that it has happened. Your tech will have to test refrigerant levels and check for leaks. The refrigerant can be recharged once any leaks are repaired.
3. Fan Malfunction
The fan blows the air across the coils to begin the heat removal and cooling process. If the fan fails for any reason, then air flow is reduced and the unit won't be able to move cold air into the home. Fans can malfunction for a variety of reasons, from broken blades and failed bearings to issues with the motor unit. The fan assembly will need to be inspected to determine the problem, then it can either be repaired or replaced depending on the type of damage done to it.
4. Frost Formation
Frost and ice can form on the coils, which then prevents air from properly moving across them and cooling. It may seem odd, but icy coils means your AC will only produce warm air -- if it blows any air at all. Many things can lead to icy coils, but typically the cause is poor airflow into the unit. Anything that impedes air flow, such as weeds growing around the exterior unit, must be removed. A unit that isn't sized correctly for the home may also be more prone to frost.
5. Weak Air Flow
Poor air flow over the coils means insufficient air will be cooled, which leads to weak air movement from the vents. The most likely cause is a dirty air filter, which is why the filter should be changed monthly. Dirt and debris blockages inside the filter housing or around the unit can also lead to air flow issues.
Contact an air conditioning repair service if you suspect issues with your evaporator coil.