Five most common FDM 3D printer faults — and how to spot them
What I see most often on the bench across Bambu Lab, Prusa Research and Creality — from clogged hotends to ringing.
Over the last year, dozens of printers have crossed my service desk in Prague — from cheap Creality Ender 3 V3 SE up to enclosed Bambu Lab X1C and Prusa CORE One. Most issues fall into five buckets. If you recognise one, it is usually an afternoon fix.
1. Clogged hotend / underextrusion
Symptom: thin walls, layers not bonding, extruder clicks or skips. On Bambu Lab P1S/X1C this shows up after PLA-CF or PETG-CF runs — the abrasive fibre wears the nozzle and carbon collects in the hot zone. On Prusa MK4S the usual cause is a contaminated Nextruder and a worn nozzle.
Fix: cold pull, replace the nozzle (hardened or ruby for abrasives), check heatbreak and PTFE.
2. Bad first layer
Symptom: poor adhesion, elephant foot, warping. Dusty PEI sheet, mis-calibrated bed sensor, or — on MK4 / MK4S — a load-cell that needs recalibrating.
Fix: clean the sheet with IPA and hot soapy water, rerun calibration, verify Z-offset on a ramp.
3. Ringing and wall artefacts
Symptom: ripples after corners, "ghosting". Input shaper not tuned, or belts loosened over time. On older Prusa MK3S+ a Klipper conversion alone cuts artefacts visibly.
Fix: tension belts, run input shaper calibration, optionally add anti-vibration dampers.
4. Electronics — thermistor, MOSFET, PSU
Symptom: mid-print failures, thermal runaway, heating drop-out. On older Ender 3 / Anycubic Kobra I most often see a burnt heatbed MOSFET. On Bambu Lab and Prusa it tends to be a loose thermistor connector or damaged cable in the drag chain.
Fix: never improvise — bad electronics is a fire risk. Replace with OEM, recheck connectors.
5. Slicer profile, not the printer
Symptom: printer is mechanically fine, prints look wrong. Around 30 % of "service" jobs I get in Prague are bad slicer profiles — wrong flow ratio, undercooled chamber, too much speed for tiny details.
Fix: tune the profile in Bambu Studio / OrcaSlicer / PrusaSlicer. There is a separate price-list item for it and it usually saves more than swapping any part.
Not sure which bucket your printer fits? Send a note via the inquiry form — online diagnostics is included.
Frequently asked questions
Usually the heatbreak, PTFE liner, or hot-end temperature is the real culprit. After a nozzle swap always run a cold pull and verify the filament flows smoothly at print speed and flow ratio.