Start here — before replacing any parts
Do not look directly at an operating UV-C bulb. With power disconnected, verify bulb seating and the exact replacement rating.
This information comes from a shared HW-series reference; check the UV module, rotor and electrical rating fitted to your HW-304B.
Manual scope: Shared HW-302/303B/304B manual
Do these checks in order
Follow the steps below and keep a record of what changes after each one. Stop when an actual cause is confirmed.
Unplug the complete filter from the wall. Identify the HW-304B nameplate and the rating printed on the UV assembly/bulb, not just its physical length.
Access the UV compartment using the HW-series instructions; inspect the lamp seating and any accessible contacts with everything disconnected. Do not run the lamp outside its protective enclosure.
If a bulb is loose but intact, reseat it fully, reclose every UV-protective cover and test using only the normal enclosed status indication provided on your exact version.
If still inoperative, fit a manufacturer-confirmed bulb of matching rating, reconnect with the enclosure closed, and use the normal covered-mode status check.
If the problem is gone
Normal enclosed UV status returns after seating or changing a bulb: that step likely addressed the failure.
If the problem is still there
A verified compatible new bulb also fails: the handbook no longer isolates the fault; a qualified service person should test the UV switch/drive circuit without exposed-lamp operation.
Which cause matches your result?
Use these more specific checks only when the earlier steps point to that possibility.
01The UV bulb is not seated correctly
Unplug the filter; only access the UV bulb as described by the manual. Check that the bulb is properly installed.
Reseat the bulb, close the protective housing and test the assembled unit.
02The UV bulb has burned out
If properly seated and still dark, the HW-series chart identifies a failed bulb as another possibility.
Install a replacement confirmed compatible with your UV assembly and electrical rating.
03The bulb still does not light
The shared troubleshooting chart does not give a complete UV power-circuit diagnostic.
Stop here and ask a qualified technician or SUNSUN support to check the UV assembly. Never test an exposed operating UV-C bulb.
How can I tell it is actually fixed?
Check the normal external indication in fully closed operation; never perform an exposed-bulb “light test.”
Before buying a replacement part
Match UV lamp wattage, socket and assembly revision before purchasing.
Where this guidance comes from
Primary reference: View the manufacturer or manual reference ↗
Applicability: Shared HW-302/303B/304B manual. Source evidence: Shared model manual confirms symptom; independent search demand not quantified.
The instructions are original English troubleshooting explanations derived from the listed technical facts; they are not a verbatim service manual. Do not apply them to an unverified revision.