Many single-phase compressors require a start capacitor to assist in starting the motor. These capacitors will occasionally fail, causing a compressor to fail to start. Overheating is a primary cause of a failed start capacitor.
Start capacitors are not designed to dissipate the heat associated with continuous operation; they are designed to stay in the circuit only momentarily while the motor is starting. If a start capacitor stays in the circuit too long, it will overheat and fail. If a failed start capacitor is discovered while troubleshooting, the start relay should be inspected as the possible cause. The contact points of the relay could be damaged, causing it to stick closed when starting and keeping the start capacitor in the circuit too long.