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Preview the sliced G-code before you press Print. The safest workflow is to inspect it in the slicer that created it, or open the file in a compatible standalone G-code viewer. Scrub through the first layer, layer changes, supports, infill, travel moves, pauses, color changes, and final layers. Then check the physical printer separately: a visual preview cannot confirm firmware state, nozzle condition, bed leveling, clearance, or the material actually loaded.
What a G-code preview can and cannot tell you
G-code is the set of movement, extrusion, temperature, homing, pause, and other commands that a printer controller executes. A preview application interprets those commands and, when available, the comments and metadata written by the slicer. It draws the expected toolpath rather than physically testing the machine.
A good preview can expose errors before they become failed prints:
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- Supports that are missing, detached, or placed where they cannot be removed.
- Unexpected gaps, thin walls, holes, or incomplete top surfaces.
- Incorrect infill pattern or density.
- Travel moves crossing areas where a collision or stringing problem is likely.
- Layer-change behavior that does not match the model.
- Pause, color-change, and custom-G-code events inserted by the slicer.
- An implausible print-time estimate or feature breakdown when the viewer can interpret the file.
It cannot prove that the selected printer profile matches the connected machine, that the firmware accepts every command, or that the printer has enough clearance, filament, and mechanical freedom. Treat the preview as a code and toolpath review, followed by a normal physical preflight.
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The complete preview workflow
- Slice the model. Load the STL or 3MF and select the actual printer, material, nozzle, and process profile. Do not begin with a generic profile if the physical machine uses different dimensions, temperatures, or start and end G-code.
- Open the preview after slicing. The slicer must finish generating the final G-code before the complete toolpath is available.
- Review the toolpath in time and by layer. Start at the first layer, move through every layer change, inspect the top surfaces and supports, and finish with the last layers.
- Check event markers and feature categories. Look for pauses, color changes, custom commands, infill, perimeters, support, and travel moves. Hide or show categories when the viewer provides those controls.
- Confirm the file context. Verify the displayed printer, print settings, and filament names against the machine and material you will actually use.
- Perform the physical preflight. Confirm the nozzle and filament, clear the build plate, set the intended bed and nozzle temperatures, check homing and leveling, and ensure the gantry and bed can move without obstruction.
- Send or start the print only after both reviews pass. A visually correct file is not a substitute for a safe machine setup.
Preview G-code in PrusaSlicer
PrusaSlicer’s built-in viewer is the most direct option when you created the file there. After slicing, switch to the Preview view. The model is replaced by the generated toolpath, with controls for inspecting layers and the sequence of movements.
Use the vertical layer selector
The vertical layer selector lets you move from the first layer to the final layer. Check the first layer at full detail: look for a continuous perimeter, complete initial infill, and any areas where the nozzle appears to travel without depositing material. Move upward at each major geometry change, around bridges and overhangs, and through the support interface. Do not inspect only the outer silhouette; many failures are visible in infill, support, or top-surface paths.
Use the horizontal time slider
Once final G-code is available, PrusaSlicer adds a horizontal slider that limits displayed extrusions and travel by time, allowing a time-based simulation. Drag it slowly from the beginning and watch the order in which the toolpath is executed. This is useful for finding an early travel move that crosses a finished wall, a pause inserted before a critical feature, or a late-stage path that extends beyond the expected model.
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Inspect feature colors and visibility controls
Use the feature legend to distinguish perimeters, infill, solid and top layers, supports, and travel. Hide categories temporarily when the full toolpath is too dense. For example, hiding infill can make a missing perimeter easier to see; hiding travel can reveal whether a surface is actually covered by extrusion rather than merely crossed by the nozzle.
Verify printer, settings, and filament labels
When those values are present in the file, the viewer displays the printer, print settings, and filament names. Compare them with the machine profile selected in the slicer and with the filament loaded in the printer. A correct-looking path generated for a different bed size, nozzle diameter, material, or temperature profile can still produce a failed or unsafe print.
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Open an existing G-code file in the standalone viewer
If the file was created elsewhere or is already stored on your computer, use the standalone PrusaSlicer G-code Viewer. Prusa Research describes it as “a lightweight application, which you can use to quickly preview G-codes from all popular slicers.” Open the application, choose the G-code file from disk, and wait for the toolpath to render before reviewing it with the same layer and time controls.
Prusa documents support for files produced by PrusaSlicer, Slic3r and Slic3rPE, Cura, ideaMaker, Simplify3D, Craftware, and KISSlicer. A file can still contain valid movement commands while lacking the comments the viewer uses to identify feature types. In that case, feature labels may be unknown or less reliable even though the geometric path is visible.
What to do when labels are missing
- Use the raw toolpath and layer controls rather than relying on feature colors.
- Check the first and final layers manually, because a missing label does not mean a missing path.
- Look for pauses, color changes, and custom commands in the file’s event markers when the viewer recognizes them.
- If the geometry appears incomplete or the file loads at an implausible scale, return to the originating slicer and regenerate the file with the correct printer profile.
Preview a file through OctoPrint
OctoPrint provides a web interface for controlling consumer 3D printers and includes a bundled GCode Viewer plugin. After uploading a file, open its viewer in the browser and inspect the layers and toolpaths before using the Start or Print control.
This workflow is useful when the file is already on the printer host. It keeps the review close to the upload action, but it does not remove the need to confirm that the connected printer is the one for which the file was sliced. Check the selected file, the printer connection, and the physical setup before starting.
How to inspect every important part of a print
First layer
Set the viewer to the first layer and examine the skirt, brim, raft, or model perimeter. Confirm that the intended adhesion aid exists and that the model’s first-layer footprint is complete. Look for isolated islands, abrupt gaps, or a large travel move where you expected extrusion. The preview cannot tell whether the bed is actually clean or level, so those remain physical checks.
Layer changes
Advance one layer at a time around changes in height. Watch for a wall that stops early, a roof that begins before its supporting geometry, or a support interface that disappears. If the slicer marks layer changes, use those markers as checkpoints rather than assuming every layer has the same pattern.
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Perimeters and thin walls
Zoom into narrow sections and corners. A wall that is thinner than the selected extrusion width may be omitted or represented by fewer lines than expected. Confirm that holes, slots, and small text remain open and legible in the path. If the preview shows no extrusion in a feature, changing the model or slicer width settings is safer than hoping the printer will reproduce it.
Infill
Check both the pattern and the density. Make sure infill reaches the surrounding perimeters and that solid layers appear beneath roofs and above cavities. Unexpectedly sparse regions, a pattern different from the one selected, or infill that terminates far from a wall can indicate a profile or model problem.
Supports and overhangs
Display support paths separately if the viewer allows it. Verify that each overhang has support where required, that support contacts the build plate or a stable model surface, and that the interface layers cover the underside that needs support. Also look for supports inside holes or channels where removal would be impractical.
Top and bottom surfaces
Inspect the final solid layers from above and below. Top surfaces should be covered by deliberate, continuous paths rather than ending with open infill. Bottom surfaces should match the intended footprint and not contain unexplained gaps. Move the time slider through the last layers to catch a top-surface path that is never reached because of an inserted pause or an earlier termination.
Travel moves
Turn travel visibility on and follow long or diagonal moves. A travel line is not extrusion, but it shows where the nozzle is expected to move. Pay attention to travels over delicate walls, across open gaps, and outside the model boundary. The preview can reveal an unexpected route; it cannot guarantee that the real machine’s gantry, clips, or cables are clear.
Pauses, color changes, and custom commands
Look for event markers at the expected heights. PrusaSlicer 2.3 and later documents comments such as ;TYPE:, ;HEIGHT:, ;LAYER_CHANGE, ;COLOR_CHANGE, ;PAUSE_PRINT, and ;CUSTOM_GCODE. These annotations help viewers identify features and events, but they are comments or metadata interpreted by software; whether a printer responds to a pause or custom command depends on its firmware and setup.
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Comparing preview methods
| Method | Best use | External files | Layer and time controls | Feature labels | Where review occurs | Can review before start? |
|---|---|---|---|---|---|---|
| PrusaSlicer Preview | Files just sliced in PrusaSlicer | Not the primary purpose | Vertical layer selector and horizontal time slider after final G-code is generated | Uses available slicer metadata and feature categories | Local slicer | Yes, before export or sending |
| Standalone PrusaSlicer G-code Viewer | G-code already on disk or made by another supported slicer | Supports PrusaSlicer, Slic3r/Slic3rPE, Cura, ideaMaker, Simplify3D, Craftware, and KISSlicer | Layer and simulation controls when the file can be interpreted | May be unknown or less reliable without expected annotations | Local viewer | Yes, before upload |
| OctoPrint bundled GCode Viewer | Files uploaded to an OctoPrint printer host | Reviews the uploaded file | Browser-based layer and toolpath inspection | Depends on what the file contains and the viewer can interpret | OctoPrint web interface | Yes, before pressing Start or Print |
The important distinction is not which viewer draws the prettiest model. It is whether the tool can open the file you have, expose the layers and movements you need to inspect, understand the originating slicer’s annotations, and let you review the file before an upload or start action.
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- Wrong printer profile: The path may fit the virtual bed but not the connected machine’s dimensions or coordinate system.
- Firmware differences: A command accepted by one firmware configuration may be ignored, interpreted differently, or rejected by another.
- Physical obstructions: Clips, cables, the frame, or a tall printed feature can interfere with movement even when the virtual toolpath is valid.
- Material mismatch: The file may specify a filament name or temperature profile that does not match what is loaded.
- Machine state: Homing, leveling, nozzle height, temperature, and sensor status are not fully validated by a static viewer.
- File-transfer errors: A file can be previewed correctly on one computer and still be incomplete or replaced after copying. Confirm the filename and the uploaded file before starting.
Troubleshooting preview problems
The viewer shows no toolpath
- Confirm that slicing finished and that you opened the generated G-code rather than the original STL or 3MF.
- Reload the file and wait for rendering to complete.
- Open the file in the standalone viewer if it was produced by another application.
- If it still fails, regenerate the G-code from the originating slicer with the correct printer profile.
The model is visible but feature colors are wrong or absent
The file may not contain the annotations expected by the viewer. Continue with layer, extrusion, and travel inspection, but treat feature names as unknown. Regenerating the file in the originating slicer can restore metadata.
The scale or bed position looks impossible
Stop before uploading. Check the selected printer, bed dimensions, origin, units, and model orientation in the slicer. A file that appears dramatically too large, too small, or outside the virtual bed is not ready for the physical machine.
The estimated time seems implausible
Time estimates depend on the commands and metadata the viewer can interpret. A missing annotation, unusual acceleration setting, or unsupported command can reduce confidence in the estimate. Use the estimate as a planning aid, not as proof that the machine will finish at that exact time.
A pause or color change does not appear
Check whether the slicer inserted the event and whether the viewer recognizes its annotation. PrusaSlicer’s documented markers include ;COLOR_CHANGE and ;PAUSE_PRINT. If the marker is absent from the generated file, it was not inserted at that point. If it is present but the printer does not pause, firmware behavior must be checked separately.
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Physical preflight after the digital review
- Install the intended nozzle and verify that it is tight, clean, and not blocked.
- Confirm that the loaded filament matches the selected material profile and is routed correctly.
- Clear the build plate and ensure the surface is suitable for the material.
- Set or confirm bed and nozzle temperatures for the selected filament.
- Home the printer and verify leveling or mesh status.
- Move the bed and toolhead through their travel range with power and operating procedures appropriate to the machine; check for clips, cables, and other obstructions.
- Confirm that the uploaded filename is the reviewed file.
- Stay available for the first layer rather than assuming a successful preview guarantees adhesion.
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If you just sliced the model
Use the slicer’s Preview view. It has the closest connection to the selected printer, material, and process settings, and it exposes the final toolpath before you export or send it.
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If the file came from another slicer
Open it in a standalone viewer that supports that slicer’s format or annotations. Expect feature names to be incomplete when the originating application does not write the comments the viewer expects. Inspect the raw path and layers instead of treating missing labels as proof that the file is invalid.
If the file is already on an OctoPrint host
Use the bundled browser viewer before pressing Start or Print. This catches a wrong upload or wrong file selection at the point where the job is about to run, but it does not replace the machine and material checks.
Frequently asked questions
Can I preview G-code made in Cura or another slicer?
Yes. The standalone PrusaSlicer G-code Viewer documents support for Cura, ideaMaker, Simplify3D, Craftware, KISSlicer, Slic3r/Slic3rPE, and PrusaSlicer files. The path may be visible even when feature labels are unknown or less reliable because the originating file lacks expected annotations.
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Open the sliced file in a slicer preview or G-code viewer and use its vertical layer control. Advance layer by layer, then use the time slider to simulate the order of extrusion and travel. Pay special attention to the first layer, changes in geometry, supports, top surfaces, and the final layers.
Does a G-code preview guarantee a successful print?
No. It validates the interpreted toolpath and some metadata, not the complete physical and firmware state. Printer selection, firmware compatibility, nozzle and filament, bed condition, temperatures, leveling, and unobstructed movement still require separate checks.
Should I inspect a file before uploading it to OctoPrint?
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