Under-extrusion is the most common print quality problem because it has six potential causes and they all produce the same symptom: not enough plastic coming out of the nozzle. Throwing parts at it without diagnosis is how you end up with a new hotend, new extruder, and the same underextruded prints. Here’s the diagnostic tree I use, ordered from most likely to least likely, so you fix it on the first attempt.
Symptom Identification: Is It Really Under-Extrusion?
Under-extrusion produces specific visual signatures. Match your print to the right signature before choosing a fix.
Perimeter gaps: Visible gaps between adjacent wall lines on the top or bottom surface. The infill looks sparse or stringy. If you can see the layer below through the top layer, you’re underextruding by at least 10%.
Thin top layers: The top solid infill shows gaps, pinholes, or barely-touching lines. At 3 top layers, you should see a fully solid surface. Holes that persist through all top layers = significant underextrusion (>15%).
Weak layer adhesion: Prints that snap along layer lines with minimal force. Properly extruded PLA at 200°C produces layer adhesion approaching 80% of the material’s bulk strength. If you can break a print along the layers with your hands, the layers aren’t bonding — typically underextrusion or low temperature.
What it’s NOT: Stringing, blobs, and oozing are over-extrusion or retraction issues. Rough vertical surfaces with regular patterns are Z-wobble or inconsistent extrusion. Irregular gaps on only certain features could be slicer settings — check your extrusion width and infill overlap.
The Diagnostic Tree: Fix It in the Right Order
Check 1: Nozzle clog or partial blockage (60% of cases)
A partially clogged nozzle still extrudes, but at reduced flow. The extrusion looks thinner, rougher, or curls as it exits the nozzle instead of dropping straight down.
- Perform a cold pull: heat nozzle to printing temperature, push filament through manually, cool to 90°C (PLA) or 140°C (PETG), then pull the filament out firmly. A clean cold pull leaves a perfect impression of the nozzle interior. Black specks, debris, or burnt filament on the pulled tip = clog confirmed.
- If the cold pull doesn’t clear it, remove the nozzle and inspect under light. A clean 0.4mm nozzle should show a perfect circle of light. A clogged nozzle shows a smaller, irregular opening.
- Replace the nozzle if cleaning fails. Brass nozzles cost $0.50 each in bulk — don’t spend 20 minutes cleaning one.
Check 2: Extruder tension (20% of cases)
The extruder gear must grip the filament firmly enough to push it against the nozzle’s back pressure. Too loose = slipping; too tight = grinding.
- Mark the filament with a Sharpie 100mm above the extruder inlet. Command a 100mm extrusion (Pronterface or LCD menu). Measure how much filament actually moved. If less than 95mm moved, the extruder is slipping.
- On spring-loaded extruders (stock Creality style), tighten the tension screw until the filament shows light gear marks but no flattening or grinding dust.
- On BMG-style dual-gear extruders, the tension is set by a spring. If the spring is weak (common on clones), replace it with a stiffer spring.
- Check the extruder gear for wear. A worn gear has a groove where the filament rides — replace the gear ($3) if the teeth are rounded or the groove is visible.
Check 3: Printing temperature too low (10% of cases)
Filament extruded below its optimal temperature is more viscous and harder to push through the nozzle. The extruder motor skips steps (audible clicking) because it can’t overcome the back pressure.
- PLA: minimum 190°C, optimal 200-210°C. Below 190°C, even a clean nozzle struggles.
- PETG: minimum 230°C, optimal 240-250°C.
- TPU: minimum 220°C, optimal 230-240°C.
- Print a temperature tower for each new filament brand. The “recommended” range on the spool is a starting point, not gospel.
Check 4: Heat creep (5% of cases)
Heat creep occurs when the heatsink fan can’t keep the cold side of the hotend cool. Filament softens above the heat break, expands, and jams. This typically happens 30-60 minutes into a print, not at the start.
- Verify the hotend fan is spinning at full speed. A dying fan spins slower, generating less airflow.
- Check the heatsink for dust buildup. A clogged heatsink can’t dissipate heat.
- For all-metal hotends: reduce retraction distance. Retracting molten filament into the cold zone where it solidifies is the primary cause. Max 4mm retraction with an all-metal hotend vs 6mm+ with a PTFE-lined.
Check 5: Filament diameter variation (3% of cases)
Budget filament can vary by ±0.05mm in diameter. A 1.70mm section where the slicer expects 1.75mm produces a 6% flow reduction — enough to cause visible underextrusion on that section.
- Measure filament diameter at 5 points along the first meter of a new spool with calipers. Below 1.72mm or above 1.78mm indicates poor quality control.
- Set the measured average diameter in your slicer’s filament settings if it varies from 1.75mm.
Check 6: Volumetric extrusion enabled by accident (2% of cases)
Some printers (notably Creality) have an “E in mm³” option in the LCD menu that enables volumetric extrusion. This changes how the firmware interprets E-steps and causes massive underextrusion. If your prints suddenly went from perfect to severe underextrusion with no hardware changes, check this setting. Disable it.
Parameter Table: Under-Extrusion Causes and Fixes
| Cause | Symptom | Diagnostic Test | Fix | Difficulty |
|---|---|---|---|---|
| Partial nozzle clog | Rough extrusion, curls exiting nozzle | Cold pull; inspect pulled filament tip | Cold pull or replace nozzle | Easy |
| Loose extruder tension | Clicking sound, filament not advancing | Mark and measure 100mm extrusion test | Tighten tension screw | Easy |
| Temperature too low | Consistent underextrusion, clicking | Print temperature tower | Increase 10°C, retest | Easy |
| Heat creep | Underextrusion starts 30+ min into print | Check hotend fan, heatsink cleanliness | Clean heatsink, reduce retraction | Moderate |
| Worn extruder gear | Inconsistent extrusion, grinding dust | Visual inspection of gear teeth | Replace extruder gear | Easy |
| Filament diameter off | Intermittent underextrusion on specific spool | Measure with calipers at 5 points | Use better filament or adjust slicer | Easy |
| Volumetric extrusion on | Sudden severe underextrusion | Check LCD menu setting | Disable “E in mm³” | Easy |
When E-Steps Aren’t the Problem
Many guides tell you to calibrate E-steps at the first sign of underextrusion. This is wrong. E-steps are a mechanical constant — they relate motor steps to filament travel distance. Unless you’ve changed the extruder motor, extruder gear, or firmware, your E-steps are correct.
Calibrate E-steps once when you build the printer or change extruder hardware. After that, under-extrusion is a flow issue (nozzle, tension, temperature), not an E-step issue. I’ve seen builders chase E-step values from 93 to 140 trying to compensate for a clogged nozzle — and then wonder why prints are dimensionally inaccurate.
As we covered in our E-Step Calibration guide, set E-steps correctly once, then use flow rate in the slicer to adjust for filament-specific behavior. The two calibration steps are related but serve different purposes.
⚠️ Safety Notice: The troubleshooting procedures described involve working with heated printer components. Wait for the hotend to cool below 50°C before handling the nozzle. Heated nozzles cause burns on contact. When performing cold pulls, the filament may release fumes — ensure adequate ventilation. Never leave a printer unattended while troubleshooting heater-related issues.
For FPV pilots who print their own parts, consistent extrusion is critical for TPU camera mounts and PETG frame parts. The UAVModel BMG Dual-Gear Extruder Upgrade eliminates filament slipping and delivers consistent extrusion even with flexible TPU. Available on our 3D printer upgrades page.