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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Start by deciding whether you have a color-transition problem or a filament-feed problem. If filament keeps extruding and the new color gradually replaces the old one, the shared nozzle probably needs a correctly configured purge. If loading or unloading stops, the filament sensor reports an error, the drive grinds filament, or no plastic emerges, increasing purge volume will not help: inspect the selector, sensors, PTFE path, filament tip, extruder and nozzle in that order.
The procedures and error terminology below are grounded in Prusa single-nozzle multi-material systems, including MMU2S/MMU3 with PrusaSlicer. A printer with separate hotends, or a different brand of feed unit, can use different purge behavior, sensors, firmware and disassembly steps. Before changing anything, identify your exact printer, MMU/AMS or other feeder, slicer and profile, firmware, nozzle, and filament.
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First separate expected purging from a mechanical fault
A shared nozzle retains a small amount of molten plastic after the previous filament unloads. The incoming filament pushes that melt through the nozzle, so the first extrusion after a change can contain the previous color. The transition clears only when enough contaminated material has been routed to a wipe tower, infill, object or another designated destination.
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That gradual color change is normal. A failure is different: extrusion stops, becomes intermittent, the feeder cannot complete a load or unload, a sensor remains triggered, filament is ground by the drive gear, or the printer reports a loading, unloading or filament-sensor error. Purge settings cannot repair a jammed selector, a false sensor state, a damaged filament tip, a slipping drive, a misaligned PTFE tube or a blocked nozzle.
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Record the symptom before changing settings
Write down the conditions of the failure while they are still visible. This prevents a purge adjustment from hiding a feed problem and makes support diagnosis much faster.
- Exact printer model and multi-material unit, such as an MMU2S or MMU3.
- Slicer, profile and firmware version.
- Material, brand, diameter and the two colors involved.
- Whether the event occurs during loading, unloading, or the first extrusion after loading.
- The complete text shown on the printer, rather than a paraphrase.
- Whether filament is visibly moving through the selector, PTFE tube and extruder.
- The slot, spool and color transition that consistently triggers the event.
- Whether the nozzle is hot and whether any plastic emerges when you command a manual extrusion.
A failure tied to one slot or one material pair is useful evidence, but Prusa error messages are not universal codes for every multi-extruder printer. Do not apply an MMU code interpretation to another manufacturer’s feeder.
Use the symptom to choose the right branch
Branch A: the new color extrudes, but the old color remains visible
Continuous extrusion with a progressively cleaner color indicates residual melt, not necessarily a jam. Check the transition’s purge volume and where the purge is being sent. In PrusaSlicer, the purge-volume matrix lets unloading and loading volumes vary by material direction. A dark pigment contaminates a light color more visibly than the reverse, so black-to-white normally needs more purging than white-to-black.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minute| Transition example from PrusaSlicer documentation | Unload volume | Load volume | Total example purge |
|---|---|---|---|
| Black to white | 80 mm³ | 70 mm³ | 150 mm³ |
| White to black | 20 mm³ | 30 mm³ | 50 mm³ |
These figures are illustrative matrix values, not universal recommendations. Your nozzle diameter, temperatures, pigments and materials can require different values. Increase only the dirty transition, in small steps, and inspect the first clean extrusion. A wipe tower, wipe to infill and wipe to object are separate destinations that can consume the purge; confirm that the selected destination is actually being generated and is large enough for the configured volume.
Branch B: loading or unloading stops
If the feeder stops, reports a sensor state, retracts unsuccessfully, or chews a groove into the filament, stop adjusting purge volumes. Begin with the filament end and the physical path. Purging happens after a successful load; it cannot overcome a filament that never reaches the extruder or a nozzle that cannot accept it.
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Step-by-step feed troubleshooting
1. Check whether the filament really cleared the feeder
For an MMU stuck-filament report, establish the filament’s location before pulling on it. If the filament has unloaded but the sensor remains triggered, inspect the selector for a broken fragment or a string of plastic. If it is caught between the MMU and extruder, inspect the full PTFE route and fittings. Debris in the selector or tube can hold a sensor or prevent the next filament from entering.
On the MMU2S jam procedure dated May 13, 2026, Prusa directs the user to release the relevant fitting, locate the jam, pull fresh undamaged filament from the spool and remove ground or damaged sections. Inspect the tip for strings, spheres and other irregular shapes, straighten the end, run the unit check, then reseat and tighten the fittings before resuming. These are MMU2S-specific steps; do not release fittings or dismantle another machine without its own manual.
2. Prepare a clean, straight filament tip
Cut away the section marked by drive teeth, bends, swelling or heat damage. The tip should be sharp enough to enter the guide without catching, but free of a loose string that can snag in a tube or selector. Prusa’s loading guides recommend cutting the tip at approximately 45 degrees for the MK4/S and MK3.9/S procedure. Keep the cut consistent when preparing multiple spools.
If a load attempt completes but no filament appears at the nozzle, the cited MK4/S procedure says to push the filament toward the extruder and retry. Do not continue pushing against strong resistance. A blocked path or hotend needs the model’s prescribed intervention, not force.
3. Inspect the PTFE path, selector and fittings
- Look for a broken fragment inside the selector or tube.
- Check for a PTFE tube that is not fully seated, sharply bent or damaged at an end.
- Inspect the filament section for parallel drive marks, deep grooves or a flattened area.
- Confirm that fittings are seated and tightened after the jam is removed.
- Verify that the selected slot corresponds to the filament path the printer is trying to use.
A damaged section can repeatedly fail even after the original jam is cleared. Cut back to clean filament before testing again.
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4. Check the extruder drive and idler
On older Prusa loading instructions, the relevant checks include a secured drive-gear pulley, clean gear teeth, correct alignment between the gear and PTFE path, and idler tension that is neither too loose nor too tight. A loose idler can slip; excessive tension can deform filament and make the next load harder. The MK4/S guide also specifies an idler-screw position after its intervention. That adjustment is model-specific and should not be transferred to another extruder.
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Use the nozzle temperature in the printer and filament profile as the starting point, then confirm it is suitable for the actual material. Prusa warns that incorrect temperatures can affect multi-material loading and unloading. Third-party filaments can behave differently in an MMU than in single-material printing, even when their nominal diameter is correct.
Moisture-sensitive filament can become stringier and leave irregular tips that catch during retraction or loading. Follow the filament maker’s drying instructions; there is no single drying temperature or time that is safe for every polymer. Do not raise temperature indefinitely to overcome a mechanical jam, because a softer filament can increase deformation in the drive path.
6. Test the nozzle only after the feed path is clear
If the selector, PTFE path, filament tip and drive appear normal but the filament still will not extrude, a clogged hotend or nozzle remains possible. Heat the printer only as specified by its manual and follow the exact model’s official cleaning procedure. A nozzle-cleaning needle may be appropriate for a documented clog, but its size and use must match the nozzle and the manufacturer’s instructions.
Never force cold filament through an unknown blockage or disassemble a hotend without model-specific guidance. Hot components can burn skin, and an incorrect reassembly can create a leak or misalignment that looks like another feed failure.
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How to set and validate purge in PrusaSlicer
Configure the transition matrix
Open the multi-material or filament settings used by the print and locate the purge-volume matrix. Treat each cell as directional: the amount for material A to material B need not equal the amount for B to A. Start with the profile’s values, then adjust the specific transition that remains visibly contaminated. A black-to-white change commonly needs more purge because a small amount of dark material is conspicuous in white.
Choose a purge destination
- Wipe tower: builds a dedicated structure that receives contaminated extrusion during changes.
- Wipe to infill: hides purge inside model infill when the geometry and material use make that acceptable.
- Wipe to object: sends purge to a designated object, which can reduce a separate tower but may affect its appearance or dimensions.
Confirm that the destination is enabled in the current print and that the generated tower or object has enough capacity. If the nozzle is still dirty despite apparently large values, check that the correct profile and material pair are active rather than multiplying the value blindly.
Calibrate conservatively
- Print a small two-color transition test using the same nozzle, temperatures and materials as the real job.
- Observe whether extrusion is continuous. If it is not, return to feed troubleshooting instead of increasing purge.
- Increase only the matrix cell for the visibly dirty direction.
- Repeat until the transition is acceptably clean, recording the value and conditions.
- Check the result at normal print speed; a value that works in a slow test may not behave identically at production speed.
Purge is a compromise between color cleanliness, print time and material waste. The example totals of 150 mm³ and 50 mm³ demonstrate directionality, not a target for every printer.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Shared nozzle versus separate toolheads
Architecture determines what “purging” means. Prusa’s explanation of its MMU3-on-MK4 change sequence describes moving over the wipe tower, rapidly ramming melted filament out, shaping the filament tip, then selecting and loading the next filament. The goal is to reduce mixed material and waste in a shared hotend.
| Architecture | What happens at a color change | Where contamination is handled | Typical fault locations |
|---|---|---|---|
| Shared nozzle with an MMU | The previous melt must be displaced before the new color is clean. | Wipe tower, infill, object or another configured purge destination. | Spool path, selector, sensor, PTFE tube, extruder or common nozzle. |
| Separate toolheads | Each hotend retains its own melt; a tool change does not automatically empty the previous hotend through the new one. | Any purge or wipe routine required by that toolhead’s design. | The selected toolhead’s feeder, sensor, hotend or nozzle. |
This is a design distinction, not a product ranking. Instructions for a tool-changing printer must come from that printer’s manufacturer and should not be replaced with MMU procedures.
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Prusa-specific procedures and their limits
MK4/S and MK3.9/S loading procedure
Use a clean tip cut at about 45 degrees. If the load finishes without filament emerging, follow the model guide’s instruction to push filament toward the extruder and retry. If the guide calls for an idler-screw adjustment, use only the stated position for that extruder. If the documented checks do not restore loading, retain the displayed error and contact Prusa support instead of repeatedly retrying.
Earlier Prusa models
The earlier “Filament Not Loading” guide emphasizes filament preparation, a secured drive-gear pulley, clean gear teeth, alignment with the PTFE path, correct idler tension and suitable temperature. It identifies a clogged nozzle as a possible cause when those checks do not resolve loading. Exact access points and adjustments vary by model.
MMU2S stuck-filament procedure
Locate the jam by releasing the appropriate fitting, remove damaged filament, inspect the tip for strings or other irregularities, straighten it, run the unit check and restore the fittings. A sensor that remains triggered after unloading points attention toward the selector and its debris state. Follow the MMU2S guide’s sequence rather than applying it to an MMU3 or another feeder.
Common scenarios and the correct response
| Observed symptom | Most useful first check | Do not do first |
|---|---|---|
| Old color fades gradually while extrusion continues | Review directional purge volume and wipe destination. | Disassemble the extruder. |
| New filament never reaches the nozzle | Inspect tip shape, selector, PTFE tube and fittings. | Increase purge volume. |
| Filament has deep drive marks | Remove damaged material and check idler tension and gear alignment. | Keep retrying with the same section. |
| Sensor says filament is present after unloading | Look for a fragment or string in the selector. | Assume the error means a clogged nozzle. |
| Path is clear but extrusion remains blocked | Use the exact model’s clogged-nozzle procedure. | Force filament or apply an unverified hotend adjustment. |
| Only one moisture-sensitive spool causes jams | Check the filament maker’s drying guidance and tip condition. | Assume every spool needs the same drying schedule. |
When to stop and escalate
Stop troubleshooting and contact the manufacturer when the documented checks fail, the same fault returns at the same mechanical location, a sensor state is impossible to clear, or the hotend needs disassembly that the model guide does not describe. Save the exact error text, note the slot and material, and record whether filament was moving. Repeated retries can further grind the filament, pack debris into the selector or obscure the original cause.
Replacement PTFE tubing or fittings are appropriate only after confirming feed-path damage and compatibility with the exact printer. Flush cutters are useful for making a clean tip. A filament dryer is relevant only for moisture-sensitive material and only according to the filament manufacturer’s recommendations. These tools do not substitute for identifying the failed stage.
What Prusa’s historical development figures do—and do not—show
In a development diary about MMU3 for the MK4, Prusa Research reported that about 15% of errors in one historical test state came from incorrect sensing of filament entering the print head. The same diary attributed roughly 50% of failures in that test state to filament not passing smoothly through the extruder into the metal filament guide. Those are manufacturer development observations, not current failure rates, universal MMU statistics or cross-brand benchmarks.
The diary also describes testing for two to three days of nonstop printing after each change to obtain a reasonable reliability sample. That is a development-method detail, not an endurance requirement for owners. No independent prevalence study establishes that the same percentages apply to another printer, firmware version, material or feeder.
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Practical decision checklist
- Color is dirty but extrusion is continuous: adjust the directional purge matrix and verify the wipe destination.
- Load or unload fails: stop changing purge; inspect the tip, selector, sensor state, PTFE path and drive.
- Filament is chewed or bent: remove the damaged section and correct idler tension or alignment for the exact model.
- Temperature or material is suspect: use the printer and filament maker’s stated ranges and drying guidance.
- Feed path is clear but plastic cannot emerge: follow the model-specific clogged-nozzle procedure.
- The error repeats after documented checks: preserve the error text and contact manufacturer support.
The Bottom Line
Purging solves residual-color contamination in a shared nozzle; it does not solve a feed fault. Diagnose the stage first, prepare a clean filament tip, inspect the selector and PTFE path, verify the model-specific extruder and material conditions, and only then clean a possibly clogged nozzle. Use Prusa procedures only on the matching Prusa model and consult the appropriate manufacturer manual for any other architecture.
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