It will be clear the UV-C bulb has blown by performing a visual test. Open the lighting housing and if the bulb is not illuminated you will need to replace the bulb.  Another indication your UV-C bulb has blown is if you notice the clarity of the water has reduced and appears cloudy or discoloured.  If you notice your water is cloudy, perform a visual test to conform you may need to replace the UV-C bulb.
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