A UV unit can look healthy from across the room. The power light glows, the housing hums, and nothing alarms. But a UV lamp loses output as it ages, and a quartz sleeve that picks up a film from the water can block light even while the lamp is still lit. The indicator tells you the unit has power. It does not tell you the lamp is delivering what the system needs.
Learn what your unit actually reports
Read the manual for the exact indicators on your model. Some units show only a ready light. Others show lamp hours, an intensity reading, or an alarm that trips below a set level. Find out which of these your unit has, what the alarm looks like, and what the manufacturer says to do when it trips. If your unit gives no output reading at all, that is a fact you should know before you rely on it.
Keep a lamp log
Write down the date the lamp was installed and the date it was last replaced. Record each sleeve cleaning and any alarm or reset. The manual sets the replacement interval, and a log lets you see when the lamp is due without relying on memory. A written history also helps a provider diagnose a problem faster.
Inspect the quartz sleeve
With power off as the manual directs, look at the quartz sleeve through the access point. A thin film, whitish scale, or a brownish coating means light is being blocked. Clean the sleeve only as the manual describes, using the method it names. Hard water and iron are common causes of this film, so a deposit that returns quickly is a sign to look at the water feeding the unit.
Watch for changes in the water
A UV unit works best on water that is clear and free of particles. If the water turns cloudy or carries sediment, the unit has more to block and the lamp may need closer attention. Note any change you see at the tap and tell the provider when it happens. Keeping the sediment filter upstream in good order also protects the UV unit.
Test the water, not just the unit
A lit indicator and a clean sleeve do not prove the water is treated the way you need. If the water is used for drinking or cooking and you have a question about it, have the water tested by a qualified lab or provider, and follow the instructions from the manufacturer and your provider. Ask the provider which warning your model gives, what it means, and what should prompt a service call.