Saturn 4 Ultra or Mars 5 Ultra: the UV light does not turn on (often due to the wrong power adapter)
Guide based on the manufacturer's official documentation; usi3d has not yet verified it on a machine.
Technology: Resin · Brand: Elegoo · Symptoms: The LCD screen or UV light does not light up · Updated on 9 October 2026
Symptom
After printing there is nothing cured on the build plate or in the resin tank, and in the exposure test the LCD shows the image but the UV light does not turn on.
Likely causes, from most to least likely
- A power adapter with the wrong rating: Mercury series adapters usually output 12 V and do not work. The Saturn 4 Ultra and 16K use 24 V and 6 A; the Mars 5 Ultra, 24 V and 3 A.
- The adapter is faulty.
- A UV light or constant-current board connector is loose.
- A faulty UV light board, constant-current board or mainboard.
Checks to tell which one it is
- Turn the adapter over and read its label: voltage and current.
- Run the exposure test: if the LCD shows the image and the light does not turn on, this is the problem.
- If you have a multimeter and experience, check that the adapter outputs about 24 V.
Before you start
Uncured resin irritates skin and eyes: wear nitrile gloves and safety glasses, ventilate the room and do not look at the UV light. Before taking anything apart or cleaning it, switch the printer off and unplug it, unless a step says otherwise. If in any doubt, stop and ask a professional.
Step-by-step fix
Prepare safely
Wearing nitrile gloves, open the UV cover and remove the build plate and the resin tank. Work in a ventilated place.
Check: You can reach the screen with no resin in the way.
If that does not fix it: Go to the next step.
Use the right adapter
Use the original adapter or one with your model's rating: 24 V and 6 A for the Saturn 4 Ultra and 16K; 24 V and at least 3 A for the Mars 5 Ultra, with the same connector. Without a multimeter, test with another such adapter.
Check: The UV light turns on in the exposure test.
If that does not fix it: Check the connectors (next step).
Check the connectors
Switch off and disconnect the adapter. Remove the seven rear panel screws (2.0 mm key) and check that the UV light and constant-current board connectors are not loose. They are held with hot glue: if they are not loose, do not remove it.
Check: With the connector seated, the light turns on in the test.
If that does not fix it: If you see a burnt connector, keep the printer off and disconnected and send photos to Elegoo support.
Measure the outputs (experienced users only)
Elegoo describes measuring with a multimeter, with the exposure test running, the voltage at the UV light connector to tell whether the mainboard, the constant-current board or the UV light board is failing (on the Saturn 4 Ultra, about 24 V at two points and about 25 V at the third). If you are not experienced, skip this step.
Check: You know which board has no output.
If that does not fix it: Contact Elegoo support with the results of each step.
Prevention
- Always use your printer's original adapter and keep it labelled.
- Do not swap adapters with other devices, for example Mercury series ones.
- Always handle the tank and resin with nitrile gloves in a ventilated place.
Still not fixed?
Tell usi3d's AI about your case: choose your printer, describe what you see and add photos if you like. It proposes checks step by step, with their sources.
Printers it applies to
Elegoo Saturn 4 Ultra, Saturn 4 Ultra 16K and Mars 5 Ultra.
Official sources
This guide summarises the official documentation linked here:
This guide is general information: usi3d has summarised it from the official documentation linked here, and it may not match your model, its version or its firmware. If in doubt, follow the manual and the support of your printer's manufacturer. You decide what you do with your machine, and working on it may affect its warranty.