What would cause a capacitor to fail?

Let us look at some of the reasons for failure of capacitors,

We summary the following reasons:

(1)Overheating

Electrolytic capacitors fail due to leakage or vaporization of the electrolyte inside.  This can be caused due to heating in operation.  Heating can be caused by either wrong connection or the use of under-rated capacitors.

In electrolytic capacitors heating can cause the formation of gas inside which can explode through the vent provided.For example,

The electrolytic capacitor contains a moist separator inside and it will dry out of the unit gets too hot, causing it to develop an internal short circuit. The capacitor can fail if its temperature routinely goes above 150 degrees Fahrenheit.

(2)Voltage surges can also cause capacitor failure.  

Overtime, capacitors re-form themselves to a particular voltage.  When an unexpected surge occurs, a failure can take place. Obviously a lightning strike from a summer thunderstorm can overload and burn out your HVAC’s electrical system., 

Also weaker power surges can cause damage to capacitors over time. 

Those weaker surges can be caused by fluctuations in the electrical grid.

Ceramic capacitors crack during over voltages.  This may create an open or short-circuit.

Tantalum capacitors are specially sensitive to voltage. 

(3)Electrical Overload

Electrolytic and Tantalum capacitors have polarity.  The leads are marked positive and negative.  Wrong polarity connections of these capacitors can cause explosion or failure. For example,

The air conditioning capacitor is designed to get the compressor and fan motors up to about 3/4 of running speed and then disengage. It is not designed for a continuous electrical load. If the motor becomes physically blocked from turning or if the motor burns out, the capacitor will probably overload and be destroyed. Voltage above the rated value for the capacitor might also destroy it. For example, a power surge from a lightning strike would cause a severe current spike and probably "fry" the capacitor.

(4)In addition to these causes, mechanical damage, heat and ageing can also cause capacitor failure.Obviously a lightning strike from a summer thunderstorm can overload and burn out your HVAC’s electrical system.,Also weaker power surges can cause damage to capacitors over time. Those weaker surges can be caused by fluctuations in the electrical grid. 

Capacitors may fail for a variety of reasons. Failure of capacitors can cause short circuits, damage to circuits, and even explosions. The same applies to starting capacitors and running capacitors

The capacitor must disengage after a few seconds or it will overheat. A compressor or fan motor that drags due to damage or worn bearings might cause the capacitor to burn up. A malfunctioning relay switch can also cause the capacitor to overheat by leaving it in the circuit too long. Lightning can damage a capacitor, a compressor, the fan motor or the wires in the unit. Even a weak power surge can damage or destroy the capacitor, which can lead to compressor overload and subsequent failure. A capacitor that's leaking oil is a sure sign that it has a problem.