There are several signs to look for if you suspect that an air conditioning compressor unit is in danger of failing. These include the compressor motor operating with more noise than usual, including buzzing and clanking sounds. Also, if a compressor has electricity but the motor and cooling fan are not running, this is a sign that there is a problem with its starter component.
Sometimes the compressor motor can "burn out," which means that the wiring inside has been burned or shorted out or the windings have been disconnected. In this case the motor will need to be replaced.
When an older air conditioning compressor has not been used for some time, or is low on oil, the compressor can become damaged and "seized." This means that the pistons can't run correctly in the cylinders.