Form 3: defects from a dirty optical window or film, or LPU errors (246, 67, 68)
Guide based on the manufacturer's official documentation; usi3d has not yet verified it on a machine.
Technology: Resin · Brand: Formlabs · Symptoms: The LCD screen or UV light does not light up, The printer shows an error code or message · Updated on 9 October 2026
Symptom
On a Form 3, Form 3+ or Form 3L, parts come out with defects because the laser arrives diffused or weakened, or the printer shows a communication error with the Light Processing Unit (LPU): Error 246, or Error 67 and 68.
Likely causes, from most to least likely
- Dust, oil, fingerprints or resin on the LPU glass optical window, diffusing or weakening the laser.
- Fingerprints, dust or dirt on the underside of the tank film, which also diffuse the laser.
- Error 246: communication failure with the LPU caused by an improper command to the galvanometer, a power issue, incorrect LPU calibration, a software bug or a damaged LPU flex cable.
- Error 67 and 68: the printer cannot communicate with the LPU.
- Dirt under the window or inside the LPU, which needs support.
Checks to tell which one it is
- Remove the build platform, the cartridge (close its vent cap first) and the tank. Tap the wrench (Settings) → Maintenance → Motor Moves and hold Left until the LPU reaches the middle; then unplug the printer.
- With the room lights dimmed and clean gloves, shine a torch or phone light at a very shallow angle from the left, front and right, looking for dust, oil, fingerprints or resin.
- Look at the underside of the tank film: fingerprints, dust or scratches.
- With Error 246, check the flex cable to the LPU, visible on the right side of its housing, with the printer unplugged.
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
Remove dust from the window
Wearing clean nitrile gloves, blow the dust off with a rubber bulb blower (better than compressed air, whose propellant contaminates the glass) and remove any large particles that could scratch it. Then dry-wipe with a new PEC*PAD, slowly, pulling it towards you, over the whole glass. Do not touch the rollers next to the window.
Check: Under the shallow light no dirt is visible any more.
If that does not fix it: If oil, fingerprints or resin remain, wet-clean it (next step).
Wet-clean the window
Only if there is oil, fingerprints or resin: put a little 90 % or stronger isopropyl alcohol (non-recycled) on a new PEC*PAD and wipe slowly, pulling towards you; repeat with clean passes until there are no streaks and finish with a dry PEC*PAD. If resin dripped on it, first soften it with a little alcohol on a clean microfibre cloth and let it dry. Do not use other cleaners: they damage the anti-reflective coating.
Check: The glass is streak-free under the shallow light.
If that does not fix it: If there is dirt under the glass or inside the LPU, or the streaks will not go, contact Formlabs support with photos of the lit window.
Clean the underside of the film
Wearing gloves, empty the resin from the tank into an opaque container, place the tank upside down on a clean surface and clean the underside of the film with a little isopropyl alcohol on a PEC*PAD. Do not clean the inside of the film with alcohol or any other solvent.
Check: The underside of the film is clean and free of fingerprints.
If that does not fix it: If the film has scratches that show on parts, use Layout in PreForm to print elsewhere or replace the tank.
Finish the maintenance
Close the cover, plug the printer in and switch it on: the LPU returns to its home position and runs a calibration.
Check: The printer starts up without errors.
If that does not fix it: If an LPU error appears, follow the next step.
Error 246, 67 or 68
Upload the diagnostic logs, do a factory reset and update the firmware and PreForm. For 246, also check that the LPU flex cable is not damaged and run the Maintenance → LPU Replacement routine without replacing the LPU: unplug when the screen asks and plug back in so it recalibrates. Formlabs has an equivalent guide for the Form 3L.
Check: The error does not come back.
If that does not fix it: If it persists, the cable or the LPU may need replacing: contact Formlabs support or your reseller.
Prevention
- Keep the cover closed when not printing: it protects the window from dust.
- Always handle optical surfaces with clean gloves.
- Before printing, check that the underside of the tank film is clean.
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
The Formlabs Form 3 family: Form 3+ and Form 3L.
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.