Troubleshooting Under-Extrusion: Causes and Fixes

Under-extrusion is that frustrating print quality problem where your printer doesn't push enough filament. The symptoms are hard to miss: thin walls with visible gaps between perimeter lines, rough and scratchy top surfaces, weak layer adhesion that makes parts snap apart with minimal force, and inconsistent line widths that make the print look like it was printed with a dying nozzle.

The tricky part is that under-extrusion has at least eight distinct causes, and the fix for each one is different. Guessing your way through the troubleshooting process wastes time and filament. Here's a systematic diagnosis approach that identifies the root cause fast.

1. Partial Nozzle Clog

This is the most common cause. A partial clog restricts flow without completely blocking it, so filament still comes out, just not enough. The usual culprit is a small piece of charred filament or debris wedged inside the nozzle bore.

Diagnosis: Manually push filament through the hot end. If it curls to one side instead of extruding straight down, or if you feel inconsistent resistance, you have a partial clog.

Fix: Perform a cold pull. Heat the nozzle to 200°C, push filament through, then lower the temperature to 90°C and pull the filament out firmly. The softened plug comes out with the filament, taking the clog with it. Repeat 2-3 times until the cold pull comes out clean with a smooth, pointed tip. If cold pulls don't clear it, replace the nozzle, they cost $1-3 and aren't worth fighting with.

2. Incorrect E-Steps

Your extruder motor has a steps-per-millimeter value (E-steps) that tells it how many motor steps push exactly 1 mm of filament. If this value is wrong, which it often is on budget printers, the motor systematically pushes too little (or too much) filament for every single print.

Diagnosis: Mark 120 mm on your filament above the extruder entrance. Command the extruder to push 100 mm. Measure how much actually moved. If only 95 mm moved, your E-steps are 5% low.

Fix: Calculate the correct value: New E-steps = (Current E-steps × 100) / actual mm extruded. Save the new value in firmware with M92 E[value] followed by M500 to store. Check our E-steps calibration guide for the full procedure.

3. Filament Grinding (Extruder Chewing)

If the extruder gear chews into the filament instead of gripping it, you get intermittent under-extrusion as the gear loses traction. Look for filament dust or shavings around the extruder gear, that's the giveaway.

Fix: Reduce extruder tension (if adjustable), clean the gear teeth with a brass brush, and check for a downstream restriction causing the gear to slip (often a partial clog). If the gear itself is worn smooth, replace it, aftermarket hardened steel drive gears cost $5-8.

4. Print Temperature Too Low

If the nozzle temperature is below the filament's optimal range, the plastic is too viscous to flow freely. The extruder motor can push it, but the resistance through the nozzle restricts actual output.

Diagnosis: Under-extrusion that worsens at higher speeds or with larger nozzles (where volumetric flow demand is higher).

Fix: Increase temperature in 5°C increments. For PLA, try 210-215°C. For PETG, try 240-245°C. Run a temperature tower test to find the optimal balance between flow and stringing for your specific filament brand.

5. Print Speed Too High

Every hot end has a maximum volumetric flow rate, the most plastic it can melt and push per second. Exceed that limit and you get under-extrusion because the filament can't melt fast enough. Stock hot ends typically max out around 12-15 mm³/s. High-flow hot ends reach 25-35 mm³/s.

Fix: Calculate your volumetric flow: layer height × line width × speed. For example, 0.2 mm × 0.4 mm × 150 mm/s = 12 mm³/s. If your result exceeds your hot end's rated flow, either reduce speed or upgrade to a high-flow hot end.

6. Bowden Tube Gap

On Bowden extruders, a gap between the PTFE tube and the nozzle inside the heat break creates a cavity where molten filament pools and pressurizes inconsistently. This causes intermittent under-extrusion that's especially visible on retraction-heavy prints.

Fix: Disassemble the hot end, cut the PTFE tube end flat with a tube cutter, and reseat it firmly against the nozzle. Tighten the nozzle while hot (hand-tight plus a quarter turn with a wrench at 200°C) to eliminate any gap.

7. Worn Nozzle

Brass nozzles wear over time, especially with abrasive filaments like carbon fiber, glow-in-the-dark, or wood-fill. The bore diameter expands from 0.4 mm to 0.5 mm or larger, changing the extrusion characteristics and causing under-extrusion at the slicer's expected flow rate.

Diagnosis: Extrude into air and measure the strand diameter. A 0.4 mm nozzle should produce strands around 0.4 mm. If strands are noticeably thicker, the bore has worn.

Fix: Replace the nozzle. Switch to hardened steel nozzles if you regularly print abrasive materials.

8. Wet Filament

Moisture-saturated filament pops, crackles, and steams as it passes through the hot end. The steam creates bubbles in the melt zone that displace plastic, causing intermittent gaps and rough surfaces that look like under-extrusion.

Diagnosis: Listen during printing. Popping and crackling sounds mean moisture. You might also see tiny wisps of steam coming from the nozzle.

Fix: Dry the filament in a food dehydrator or filament dryer at 45-55°C for 4-6 hours (PLA/PETG) or 6-8 hours (Nylon/TPU). Store dried filament in sealed containers with desiccant.