The evaporator coil in your AC serves a very important function—it is the device that actually extracts heat from the air so it can be cooled by the AC. If the coil fails, your AC won't work properly.
Signs of Problems
There are several signs that something is wrong with the evaporator coil. Most alarming is that the air coming through the AC vents and into your home will be warm at best, and hot at worst. Since the blower isn't affected, the air still circulates but it can't be cooled.
Your AC may also begin to cycle frequently, which means it turns on again almost as soon as a cycle ends. This is because the house is not cooled to the thermostat setting since the evaporator coil is failing. Eventually, this may cause the AC to overheat and go into an emergency shut down mode, which means it will stop running completely.
Two main issues affect the evaporator coil. The first is dirt. If too much dust, dirt, and debris get into the exterior AC unit, it can coat the evaporator coil. This in turn prevents the coil from effectively performing heat exchange because the dirt acts like an insulative layer.
The other issue is a refrigerant leak. Without refrigerant in the lines, the coil can't properly perform its heat exchange duties and cool the air. Sometimes you may notice ice build-up on the coils—this is one sign of a refrigerant leak. Of course, ice build-up can also be caused by an improperly sized AC, so further inspection by an AC repair service is necessary to verify the root of the problem.
If the coil is simply dirty, then repair is little more than scheduling routine maintenance. The coil, along with the rest of the air conditioner, will be thoroughly cleaned and inspected for damage. Ideally, the exterior unit and coil should be cleaned annually in the spring. More frequent cleaning may be necessary if your area is particularly dusty, though.
A refrigerant leak is a more involved issue. Sometimes the leak can be repaired, usually by replacing the damaged line or the evaporator coil. Then, the unit is recharged with some refrigerant. Depending on the age of the AC and whether it is sized correctly, your tech may recommend replacing the unit rather than repairing the leak.
Contact an air conditioning repair service for more help if you suspect issues with the evaporator coils.