The compressor in a central air conditioner runs when the system needs refrigerant to keep pumping through to facilitate cooling. When your indoor temperatures reach the level set at the thermostat, the compressor and thus the system is supposed to shut off. But issues in the system can leave the compressor running for longer period of times than normal. The problem can tie back to the compressor itself but can also occur due to some common problems elsewhere in the system.
Dirty or Frozen Coils
Your central air conditioner has two sets of coils that help move the refrigerant through the system. Condenser coils in the outside condensing unit take in gas refrigerant then convert the refrigerant to a liquid that can move through the pipes into the house. Inside the house, evaporator coils in the air handler convert the refrigerant from a liquid to a gas.
Dirt on either set of coils can impair the coils' ability to efficiently convert the refrigerant. The inefficient refrigerant will cause the evaporator coils to become less cold during their conversion process, which will then produce less cool air for your home. The air conditioner will keep running to try and get the air cold enough but the continued refrigerant inefficiency make that difficult. The evaporator coils can also become frozen over due to an inadequate supply of refrigerant and the freezing has a similar result as dirty coils.
You can clean the coils using a commercial coil cleaner but it might be a better idea to call in an air conditioning services company to clean the coils and top off the refrigerant, if necessary.
Low or Leaking Refrigerant
Coil dirt isn't the only cause of inefficient cooling. Low levels of refrigerant can occur due to the passage of time or a leak in the coils or piping. The refrigerant can also require a recharging, which makes the existing refrigerant more efficient.
If you suspect a refrigerant leak, turn off your unit and call for an HVAC service call as soon as possible. Refrigerant is a chemical that you don't want to handle without proper safety equipment. And only a licensed HVAC technician can buy the refrigerant to top off your levels, change out your existing refrigerant for a different type, or to charge your refrigerant.
Blower Fan Issues
This problem again traces back to the coils. The refrigerant in the condenser coils makes the exterior of the coils warm. A motorized fan circulates air through the unit so that the coils efficiently change the refrigerant and so that the coils don't heat up so much the system overheats and shuts down. Problems with the fan can cause the coils to inefficiently change the refrigerant, which can lead to the system running for longer to cool your home.
You can perform a quick check for fan issues by turning on your system and then standing next to the condensing unit. Do you hear a fan spinning at all? Is the fan spinning but not as fast as it used to? There could be a problem with the blower fan or its motor. Call in an air conditioning contractor service for diagnostics and replacement parts.