Many, but not all spas, come with UV sterilization systems. Typically there is one only thing that goes wrong with UV systems and that is the Ultraviolet Lamp.
With most systems, failure of the ultraviolet lamp will be indicated by shrill whistle coming from under your spa. This is your indication that it is time to replace your UV lamp. Once you open the cover to your spa you may find an additional indicator light.
Please review the video below showing the process of changing the UV bulb: