Your print has gaps between perimeter lines, layer adhesion is so weak the part snaps with finger pressure, and the extruder makes a clicking sound every few seconds. This is under-extrusion — the printer is delivering less filament than the slicer expects. Here’s the systematic diagnosis that finds the root cause in 15 minutes instead of replacing parts at random.
How to Confirm It’s Actually Under-Extrusion
Before tearing anything apart, verify the problem is under-extrusion rather than a slicer issue. Load a calibration cube sliced with 2 perimeters, 15% infill, at your normal print settings. If the cube shows:
- Gaps between perimeter lines: Filament isn’t filling the toolpath width
- Top/bottom layer gaps between parallel lines: The most definitive sign of under-extrusion
- Weak layer adhesion: Layers peel apart with moderate force because contact area is reduced
- Thin, inconsistent extrusion width: Individual extrusion lines vary in thickness
- Extruder clicking/skipping: The stepper motor loses steps because it can’t push filament through
If you see all of these, it’s under-extrusion. If you only see weak adhesion but lines look solid, check your printing temperature — too-cold filament bonds poorly even when extrusion volume is correct.
The Systematic Diagnosis Sequence
Start at the nozzle and work backward. The nozzle is the most common failure point and the fastest to check.
Step 1: Nozzle Clog — Manual Extrusion Test
Heat the hotend to your normal printing temperature. Manually push filament through by hand (release the extruder tension arm first). If filament comes out straight and consistent, the nozzle is clear. If it curls, comes out thin, or requires significant force, the nozzle is partially clogged.
Fix: Cold pull (atomic pull). Heat to 170°C for PLA (140°C for PETG), push filament in, let it cool to 90°C, then pull firmly. The solidified filament tip captures debris from inside the nozzle. Repeat 2-3 times until the pulled tip is clean. If cold pulls don’t clear it, replace the nozzle — brass nozzles are $1-2 and not worth fighting over.
Verification: After clearing, manual extrusion should produce a straight, consistent filament stream that drops directly down without curling.
Step 2: Heat Creep — The Sneaky One
Heat creep happens when heat travels up the heat break into the cold zone, softening filament before it reaches the melt zone. The softened filament expands, increases friction against the heat break walls, and eventually jams.
Symptoms: Print starts perfectly, then under-extrudes progressively over 30-60 minutes. The extruder clicks. The failure is time-dependent — shorter prints complete, longer ones fail.
Diagnosis: Touch the top of the heatsink (where the heat break enters). After 30 minutes of printing, it should be cool or slightly warm, not hot. If it’s uncomfortably warm to touch (above 50°C), you have heat creep.
Fixes (in order of likelihood):
1. Hotend fan not running or running slow — verify the fan spins at full speed
2. Hotend fan airflow path blocked by dust — clean the fan and heatsink fins
3. Ambient temperature too high (enclosure without exhaust) — add an exhaust fan or open the enclosure door for PLA
4. Retraction distance too long on all-metal hotend — reduce retraction to 3mm or less
5. Thermal paste on cold-side threads dried out — disassemble, reapply thermal paste
Verification: After applying the fix, print a 2-hour test object. If it completes without underextrusion, heat creep was the cause.
Step 3: Extruder Tension and Gear Wear
The extruder has two jobs: grip the filament and push it consistently. If either fails, you get underextrusion.
Check extruder tension: The tension arm spring should compress the idler bearing firmly against the filament. When you release the arm, it should snap back with noticeable spring force. If the spring is weak or the arm pivot binds, the filament slips.
On plastic extruders (stock Ender 3): The tension arm develops hairline cracks near the pivot point. The crack is invisible when the arm is at rest but opens under tension, reducing grip. Remove the tension arm and inspect the underside with a bright light. Any crack — replace with an aluminum extruder ($10-15).
Check drive gear: The brass drive gear (hobbed gear) wears over time. Under a magnifier, the teeth should have sharp, defined edges. Worn teeth are rounded and polished smooth. A worn gear slips on the filament under back pressure. Replace every 500-1000 print hours or when the teeth look rounded.
What happens if you get it wrong: Continuing to print with a cracked tension arm — the crack propagates, the arm snaps completely mid-print, and filament stops feeding entirely. The extruder motor spins but no filament moves. A $10 aluminum extruder prevents this forever.
Step 4: E-Step Calibration
E-steps tell the firmware how many stepper motor steps correspond to 1mm of filament. If e-steps are off by 10%, every print underextrudes by 10%.
Calibration method:
1. Mark the filament 120mm above the extruder inlet (measure with calipers)
2. Command the extruder to feed 100mm: G1 E100 F100
3. Measure remaining distance from mark to inlet
4. New e-steps = (current e-steps × 100) / (120 – remaining distance)
5. Set via M92 E<new_value> and M500 to save
Run this twice — if the second measurement shows exactly 20mm remaining (100mm fed), the calibration is correct.
What happens if you get it wrong: Under-calibrated e-steps (extruder thinks it fed 100mm but only fed 90mm) produce a consistent 10% under-extrusion on every print, regardless of temperature or speed. This is the easiest problem to fix but surprisingly many people skip it.
Step 5: Volumetric Flow Limit
Every hotend has a maximum volumetric flow rate — the volume of filament it can melt per second. Push beyond this limit and the extruder skips because unmelted filament can’t pass through the nozzle.
Standard Ender 3 / CR-10 hotend: 10-12 mm³/s (PLA at 210°C)
E3D V6 / all-metal: 12-15 mm³/s
Volcano / high-flow: 20-25 mm³/s
Rapido / Revo High-Flow: 25-35 mm³/s
Calculate your actual volumetric flow: layer height × extrusion width × print speed.
Example: 0.2mm height × 0.45mm width × 80mm/s = 7.2 mm³/s. This is well within a stock Ender 3’s capability.
Example: 0.28mm height × 0.6mm width × 120mm/s = 20.2 mm³/s. This exceeds a stock hotend’s limit — the extruder will skip.
Fix: Either reduce speed, reduce layer height, or upgrade the hotend. If you need 20+ mm³/s consistently, a high-flow hotend is required.
Under-Extrusion Quick Diagnosis Matrix
| Symptom | Most Likely Cause | Secondary Cause | Fastest Fix |
|---|---|---|---|
| Gaps in top/bottom layers | E-steps wrong | Partial nozzle clog | Calibrate e-steps, cold pull |
| Extruder clicking | Nozzle clog | Heat creep | Cold pull, then check fan |
| Good start, fails after 30+ min | Heat creep | Extruder gear worn | Clean fan, check heatsink temp |
| Consistent on all prints | E-steps wrong | Flow rate in slicer wrong | Calibrate e-steps |
| Only on fast prints | Volumetric flow limit | Temperature too low | Slow down or raise temp 10°C |
| Pattern repeats every few mm | Damaged extruder gear | Filament diameter variation | Replace drive gear |
| Only on first layer | Nozzle too close to bed | Bed not level | Adjust Z-offset |
| Only after filament change | Filament diameter off | Different melt temp needed | Measure filament, temp tower |
| Intermittent clicking | Cracked tension arm | Loose grub screw on gear | Replace with aluminum extruder |
What Most People Get Wrong
Mistake 1: Jumping to e-step calibration without checking for a physical problem first.
If the extruder is clicking, e-steps are not the problem. Clicking means the stepper motor is losing steps because it encounters physical resistance. Changing e-steps when the extruder is skipping just means the firmware now expects different step counts — the motor still skips. Fix the mechanical issue, THEN recalibrate e-steps if needed. The sequence is: nozzle → heat creep → extruder mechanics → e-steps. Skipping steps creates false diagnoses.
Mistake 2: Printing a temperature tower without addressing the underlying mechanical problem.
A temperature tower shows that the printer underextrudes less at 220°C than at 200°C — so the natural conclusion is “I need to print hotter.” But if the real problem is a partially clogged nozzle, printing hotter masks the symptom while the clog continues to worsen. In three more prints, even 230°C won’t push filament through. Diagnose the mechanical cause first. Temperature is the last variable to change, not the first.
Mistake 3: Ignoring filament diameter variation.
“1.75mm” filament from budget manufacturers varies from 1.68mm to 1.82mm. If your filament averages 1.70mm and your slicer assumes 1.75mm, the actual cross-sectional area is 5-8% less than expected — producing consistent, baffling under-extrusion. Measure filament diameter at 5 points over 2 meters with calipers. If variation exceeds ±0.03mm, either adjust the slicer filament diameter or switch to a higher-quality brand. As discussed in our filament dryer guide, moisture also affects extrusion consistency — wet filament pops and sputters in the hotend, producing intermittent gaps that look like under-extrusion but aren’t.
Mistake 4: Replacing the hotend when the problem is a cracked extruder arm.
The classic: “My under-extrusion is so bad, I need a new hotend.” Remove the extruder tension arm first — a $2 part with a hairline crack that’s invisible when installed. If the arm is cracked, the idler bearing doesn’t press firmly enough against the filament. The drive gear spins against the filament, grinding a divot. Replace the arm first, then evaluate whether the hotend actually needs attention. Stock Ender 3 plastic extruders crack within 3-6 months of regular use. It’s a design flaw, not a manufacturing defect — the pivot point is too thin for injection-molded plastic.
Mistake 5: Running the same print speed after switching to a larger nozzle.
A 0.6mm nozzle at 0.3mm layer height and 80mm/s demands 40% more volumetric flow than a 0.4mm nozzle at 0.2mm and 80mm/s. The stock hotend that handled 0.4mm perfectly now skips because it can’t melt filament fast enough. When increasing nozzle size, calculate volumetric flow first and adjust speed accordingly. For nozzle sizing fundamentals, see our 3D printer nozzle size comparison guide.
⚠️ Safety Notice: When performing cold pulls, the hotend is heated — avoid contact with the nozzle and heater block. If using a brass brush to clean the nozzle, turn off the printer first — a shorted thermistor or heater cartridge can damage the mainboard. Always ensure thermal runaway protection is enabled in firmware.
For extruders that eliminate the cracked-arm problem permanently, the Micro Swiss dual-gear extruder uses hardened steel drive gears and an all-aluminum body. Unlike single-gear extruders that rely entirely on spring tension for grip, dual-gear systems drive filament from both sides — cutting the required spring tension in half while doubling grip force. Available at uavmodel.com for Ender 3, CR-10, and similar open-source printers.
Teaching Tech’s under-extrusion diagnosis guide covers the systematic approach with practical demonstrations of each failure mode: