An ultraviolet disinfection unit can keep running, its power light can stay on, and its output can still drop. The lamp ages, the quartz sleeve around it can foul, and water that is cloudier than expected upstream can shield the lamp from the light it needs. This guide shows what to check so you are not relying on a light that only confirms the unit has power.
Start with the lamp replacement record
Find the replacement interval in the manual or on the unit, then find out when the lamp was last changed. Many owners never write this down, so the first task may be reconstructing the history from the service invoice. If the lamp is past its recommended interval, treat that as the first problem to solve, even if the unit looks fine.
Ask whether the unit has an hour counter or a service reminder. If it does, check whether it has been reset after the last lamp change. A counter that was never reset tells you nothing useful.
Look at the lamp and the sleeve
With the power off and the manufacturer's safety steps followed, check the lamp for dimming, discoloration, or an uneven glow along its length. The sleeve should be clear. A film, scale or mineral coating on the sleeve reduces the light that reaches the water, even when the lamp is new. If you are not comfortable opening the housing, leave this to the service provider and ask them to report what they found in writing.
Check the water before the lamp
UV treatment works only on water that light can pass through. Cloudiness, tint or sediment in the water entering the unit reduces its effectiveness. Look at the pretreatment filter, if the system has one, and find out when its cartridge was last changed. A clogged or overdue pretreatment filter can make a healthy lamp look like a failing one, or hide a real lamp problem behind a water problem.
Ask whether the unit has a sensor or alarm
Some UV units include an intensity sensor or an alarm that signals low output. Ask whether yours has one, whether it is enabled, and how it is tested. A sensor that has never been checked, or an alarm that has never been triggered on purpose, cannot be trusted to warn you. Ask the service provider to test the alarm and record the reading.
What to ask the service provider
Ask them to confirm the lamp age, the sleeve condition, the pretreatment status, and whether the output is being measured with a meter or only inferred from the indicator. Ask for the replacement interval in writing and when the next check should happen. A clear written answer protects you whether the unit turns out to be fine or needs a new lamp.