Troubleshooting Extruder and Hotend Problems with the Prusa i3 Mk3


These notes apply to the Prusa i3 Mk3 printer. If you are using a different printer, please verify the hardware details are same. These pages may be a bit rough as I revise them and add new material. Please check back regularly for updates.

Common extruder and hotend problems include:

  • Extruder clicks during printing.
  • Extruder skips or bucking during printing.
  • Filament jams and clogs.
  • Erratic under-extrusion.
  • Excessive extruder motor temperatures (too hot to touch).

The following is a list of troubleshooting steps that I’ve put together after assisting and observing many Mk3 owners dealing with these problems on the Prusa forums and Reddit discussions.

Under normal conditions, the “heatbreak” – a small space between the cold heatsink and heated block – is sufficient to keep filament from softening prematurely. If heat creeps into the cold zone, either due to hot ambient temperatures, buildup of heat from the extruder motor, or from excessive retraction pulling molten filament to far up, problems can occur.

Troubleshooting questions

Prusa technical support is excellent, but you can speed things up by doing some preliminary work before contacting them. I suggest working through this list and at least being able to answer these questions ahead of time:

Have you added any fan or extruder enhacements or decorations?

Thingiverse is full of neat fan covers and other customizations that may significantly impede cooling. In many cases, these additions are decorative, or of questionable function.

  • If you’re using fan covers, grilles or other goodies, try taking them off.
  • If you’re using a filament guide or cleaner, make sure it’s not interfering with the feed. Remove for testing.

Is your filament feed path free from partial clogs?

Raise Z to max and extrude material

  • Does filament flow cleanly from the nozzle, or angle to one side?
  • Does extrusion look normal?

If the filament does not feed cleanly, do some cold pulls to remove any crud build-up in the nozzle and hot end.

Are you printing in unusually warm ambient temperatures?

Increased temps can contribute to feed problems. The E3D V6 extruder operates best near 40C ambient room temps.

Cool or increase airflow if necessary.

Are you printing in an enclosure?

Enclosures can impede airflow and increase ambient print temperatures. See above.

Are you running current firmware?

Recent firmware updates (> 3.2.1) address extruder motor temps by adjusting motor currents. Are you running current firmware?

Are you printing from anywhere but directly off of SD card?

Octoprint or other server software can cause problems if the hosting server (Raspberry Pi?) gets too busy. Try printing from SD card directly to see if the problem still occurs.

Are your extruder tensioning screws set properly?

  • Is the filament visibly distorted or shredded? Too tight and the filament can be distorted and snag or jam, causing the extruder to do extra work and heat up.
  • Is the tension too loose? Too loose and filament doesn’t feed, causing the extruder gears to slip, resulting in clicks and jams.

The screws normally protrude < 1mm from the left side extruder housing cover.

Is your extruder idler rotating freely?

Open the extruder right cover and verify.

  • Does the idler on the right cover should spin freely without friction?

If the idler gear on the right cover does not spin freely, feed problems and jams can occur. Verify the gear is properly inserted into the guides, and that the geared assembly spins freely. Refer to the maintenance manual if not.

Is area around the Bondtech extruder gears clear of filament and crud?

With the right extruder cover open, inspect with a flashlight.

  • Is the extruder area clear of debris?

A rusty-looking smattering of dust is normal, but loose filament and other material is not. Blow it out, clear it with a brush if necessary.

  • Is the white PTFE tubing protruding from below the gears damaged?

Replacement or reshaping of the PTFE tube may be required.

Does filament feed cleanly down through the top, through the gears and into the PTFE tubing below the gears?

Make sure there are no jams or twists. A 6”/150mm length of 1.5mm rod with rounded tips (to avoid damaging the PTFE tubing) can be useful for testing. A 1.5mm hex wrench will work in a pinch. With the extruder gears loose and filament removed, slip it in through the top and examine as it feeds past the loosened extruder and into the PTFE tube below.

  • Does the rod move cleanly through:
    • The top feed?
    • Down past the (loosened) extruder gears?
    • The PTFE tube below the extruder gears?
  • If you’ve removed the nozzle, does it cleanly poke through the bottom of the heater block?

Examine the PTFE tube for damage. Make sure there are no snags. If you’ve removed the nozzle, the rod should slide through without resistance. Be careful not to snag or damage anything.

Tip: This same technique can be used from above to clear PTFE clogs, or from below to clear filament that breaks off below the extruder but above the hot end.)

Are you using appropriate retraction settings?

Retractions pull the filament up from the hotend to relieve nozzle pressure. This can help with oozing and stringing, but can also contribute to heat-related problems.

  • Are you using excessive retraction distances? The Prusa i3 Mk3 uses a direct feed mechanism. Many guides recommend retraction settings of 5mm or more which is appropriate for a Bowden setup, but not the direct feed mechanism of the Prusa i3 Mk3. If your retraction length is above 2mm, you are pulling melted material back up past the heat break. Keep retraction distances around 0.8mm.
  • Are you using overly-aggressive retraction settings? Does the point at which your print fails consist of lots of small areas (e.g. fingers) and retractions? Preview the sliced model in your slicer software to see if you have lots of retractions occurring in small areas. Increase the minimum travel required to trigger retractions as a possible troubleshooting step. Increase the minimum travel required to trigger retractions as a possible troubleshooting step. If using Slic3r Prusa Edition, Prusa sets the minimum move to 1mm. Slic3r defaults to 2mm. Try 5mm. You may get some stringing, but can deal with that separately.

Do you experience failures with one material over others?

Increasing temps slightly may improve flow. It might also contribute to unwanted heat.

Are you exceeding the maximum volume capacity (max volumetric speed) of the E3D V6 extruder

The E3D V6 hotend used on the Prusa i3 Mk3 can process roughly 11.5 mm3/s. Any attempts to move filament any faster through the hotend can result in backpressure on the extruder or jams in the nozzle.

  • Does slowing the print down from the front panel (say to 50%) reduce problems?
  • Does adjusting slicer speed or volumetric feed settings reduce problems?

Prusa’s provided speed settings for most slicers are extremely aggressive. Their settings for Slic3rPE set the flow rate for PLA to 15 mm3/s , which exceeds the E3D V6 capacity. Other materials are set to 1-10 mm3/s , which may explain why PLA fails and others work. Check your slicer preview for flow rates. Reduce max volumetric speeds if using Slic3rPE. You may be stuck just reducing speeds with other slicers.

Proactively Avoiding Problems

You can do some analysis to avoid many of these problems and judge the effectiveness of these measures yourself.

  • Slice your model in Slic3rPE, save the gcode file.
  • Click on the Preview button at the bottom.
  • Select Volumetric flow rate in the drop-down box at left. Do you see it hitting or exceeding the 11.5 mm3/s mark at the places it commonly fails?
  • Select Feature types on the drop-down and enable showing Retractions with the checkbox. Do you see a lot of retractions at the failure points?
  • Select Speed in the drop-down and go through the layers until you hit the common failure point. Are speeds high at these points?

This analysis might help pinpoint why you’re having these problems short of replacing hardware (which may or may not fix the problem).

Pesistent problems

In some cases, the measures listed above are not sufficient. Here are some more involved troubleshooting and work-around steps.

Dealing with Filament Blobs

Filament blobs can form inside the extruder mechanism when hot filament is pulled up into the extruder area and cools. These can sometimes be large enough to prevent removal of the filament through the top cover, requiring opening the extruder cover and snipping the filament.

One tip for avoiding blobs is to heat up the hotend and extrude a small amount (1 inch/25mm) of material before ejecting filament.

Ideally, you will be able to eliminate blobs from occurring using the steps listed above, but there are printable covers on Thingiverse that unscrew or swivel to allow removing the blobs without cutting.

Applying Thermal Paste to the Heatbreak

Some users have reported success fixing nagging extruder problems by applying thermal paste between the threads on the heatbreak and heatsink.

Improved Extruder Parts

Prusa has issued updated Prusa R3 parts designed to improve cooling. I personally have not updated as of this writing due to concerns about airflow and increased noise levels with the new parts.

Contact and feedback

You can find me on the Prusa forums or Reddit where I lurk in many of the 3D printing-related subreddits.

Last updated on Oct 23, 2018