How to Fix Linear Advance: Common Issues and Solutions

Linear Advance (LA) is a Marlin firmware feature that adjusts extrusion pressure to minimize stringing, blobbing, and under-extrusion during direction changes. While it drastically improves print quality when tuned correctly, misconfigurations or hardware issues often lead to frustrating print defects. Below are the most common Linear Advance problems and step-by-step fixes.

Prerequisites for Troubleshooting

Before diving into fixes, confirm you have:

• Marlin 2.0 or later firmware with Linear Advance enabled (M900 command supported)
• A calibrated Linear Advance K-value for your filament (run the official LA test pattern first)
• Functional extruder with no mechanical binding, proper stepper current, and correct steps/mm calibration

Common Issue 1: Under-Extrusion on Acceleration/Deceleration

Symptoms: Gaps in walls, missing layers, or thin extrusion when the print head speeds up or slows down.

Causes

• K-value set too high for your filament/hardware
• Extruder steps/mm not calibrated (under-counting extrusion distance)
• Extruder motor skipping steps due to low current or binding

Solutions

• Reduce your K-value by 0.05 increments using M900 K[new value], then re-run the LA test pattern to verify.
• Recalibrate extruder steps/mm: Mark 100mm of filament, extrude 100mm via G1 E100 F100, measure remaining filament, and adjust steps/mm using M92 E[new value].
• Increase extruder stepper current in your firmware (or via LCD if supported) by 5-10% to prevent skipping, ensuring the motor does not overheat.

Common Issue 2: Blobbing or Over-Extrusion at Corners

Symptoms: Excess filament buildup at sharp corners, layer shifts, or raised blobs on print surfaces.

Causes

• K-value set too low for your filament
• Junction deviation or acceleration settings too high (exceeding extruder capacity)
• Filament diameter variance (using filament larger than the 1.75mm/2.85mm set in your slicer)

Solutions

• Increase K-value by 0.05 increments via M900 K[new value], then re-test.
• Lower acceleration settings in your slicer (start with 500mm/s² for PLA, 300mm/s² for flexible filaments) or reduce junction deviation to 0.1mm or lower.
• Measure your filament diameter at 5 points along 1m of filament, set the average in your slicer, and enable diameter variance compensation if available.

Common Issue 3: Linear Advance Not Taking Effect

Symptoms: No change in print quality after adjusting K-value, or M900 command returns an error.

Causes

• Linear Advance not enabled in Marlin firmware (LIN_ADVANCE disabled in Configuration.h)
• Using a slicer that overrides firmware extrusion settings (e.g., Cura's "Experimental" extrusion adjustments)
• K-value not saved to EEPROM (lost on printer reboot)

Solutions

• Recompile Marlin with #define LIN_ADVANCE enabled in Configuration.h, then flash the updated firmware.
• Disable slicer-specific extrusion modifiers: In Cura, turn off "Flow Compensation" and "Pressure Advance" (if using Cura's built-in version instead of Marlin's LA). In PrusaSlicer, disable "Pressure Advance" in the Extruder settings.
• Save your K-value to EEPROM after setting it: Run M900 K[your value], then M500 to save, and M501 to load on boot.

Common Issue 4: Stringing Persists Despite LA Tuning

Symptoms: Thin strings between printed parts even with a calibrated K-value.

Causes

• Retraction settings too low (LA does not replace retraction, it complements it)
• Print temperature too high for your filament
• Insufficient travel speed (slow moves allow filament to ooze)

Solutions

• Adjust retraction settings: Increase retraction distance by 0.5mm (max 2mm for direct drive, 6mm for Bowden) and retraction speed by 10mm/s, then re-test.
• Lower print temperature by 5-10°C increments until stringing stops, ensuring layer adhesion is not compromised.
• Increase travel speed in your slicer to 150mm/s or higher for non-perimeter moves to reduce oozing time.

Hardware Checks for Persistent Issues

If software fixes fail, inspect your hardware:

• Extruder gears: Clean debris, check for worn teeth, and ensure proper grip on filament.
• Bowden tube (if applicable): Cut the tube flush to the nozzle and extruder fitting, ensure no gaps that cause filament to expand.
• Nozzle: Replace clogged or worn nozzles (0.4mm standard nozzles wear out after ~1kg of abrasive filament).

Final Tips

Always test one change at a time to isolate the root cause. Linear Advance K-values vary by filament type (PLA, PETG, TPU all require different values) and even by spool, so re-calibrate when switching materials. Keep a log of K-values per filament brand and type to speed up future tuning.