3D printer not printing: diagnosing and fixing common faults

Your 3D printer powers on, the bed homes, the slicer shows the print running — and nothing useful comes out. Or it starts fine and fails halfway through. Either way, there are only so many things that cause this, and most of them are diagnosable in an hour.

The nozzle is blocked

This is the most common culprit. The extruder motor clicks or skips, or you get a thin thread of filament instead of a solid line.

Heat the nozzle to your normal print temperature and try pushing filament through by hand. If it resists, you have a partial clog. Nothing moving at all means a full blockage.

A cold pull will fix most clogs:

1. Heat the nozzle to around 240°C — higher than your normal print temperature. 2. Push fresh filament through until it flows freely. 3. Drop the temperature to 85–90°C for PLA, or around 110°C for PETG. 4. When it reaches that lower temperature, pull the filament out in one firm, steady tug. 5. Look at the pulled tip — it should be shaped like the inside of your nozzle, with any debris embedded in it. 6. Repeat until the tip comes out clean.

If cold pulls are not shifting it, a 0.4 mm acupuncture needle pushed into the tip from below can dislodge compacted material. Heat the nozzle first, then probe carefully. Too much force and you will damage the nozzle.

Sometimes the blockage sits in the heatbreak — the narrow throat between the hotend body and the cold zone. PTFE-lined heatbreaks accumulate debris over time, especially if the printer has been run above the PTFE's rated temperature. If cold pulls come out clean but extrusion is still weak, the heatbreak is worth inspecting next.

The extruder is clicking or grinding

That click-click-click from the extruder means the drive gear is slipping on the filament. Either it is fighting a downstream blockage, or the filament itself is the problem.

Check the idler tension first. Most direct-drive and BMG-style extruders have a spring or adjustment screw that sets how firmly the gear grips the filament. Too loose and it slips. Too tight and it grinds a groove in the filament. You want it snug — firm resistance when you tug the filament, but no grinding dust forming on the drive gear.

Then check the filament. Filament left open in humid conditions absorbs moisture and becomes brittle. It will snap under the stress of feeding, and a short stub left in the extruder body will jam the path completely. Dry affected filament at 45–65°C for four to eight hours before use. Snapped stubs usually need picking out with a pin before you load fresh filament.

The first layer is not sticking

The print starts but peels away from the bed, or the nozzle drags the first layer around and makes a mess. Work through these in order.

Z offset. The nozzle needs to be close enough that it squishes the first layer slightly into the bed surface. Too high and the lines are round on top — they will not bond to the bed. Too low and the nozzle scrapes or completely blocks the gap. Run your printer's levelling routine, then adjust the Z offset live during a first layer print until the lines look flat, like a ribbon pressed onto the surface.

Bed temperature. PLA typically needs 55–65°C. PETG wants 70–85°C. ABS needs 100–110°C and a draught-free enclosure. If your thermistor is reading inaccurately, the bed will look correct on screen while actually being cold. This tends to show up as prints that suddenly start failing after months of reliable use.

A dirty bed. PEI, glass, and Garolite all lose adhesion if there is finger grease on the surface. Wipe with 90% or higher isopropyl alcohol before each print. If IPA alone is not enough, wash with a drop of washing-up liquid on a damp cloth, rinse with clean water, then finish with IPA. Do not use acetone on PEI spring steel sheets — it will damage the coating.

It starts printing, then stops

Mid-print failures have a different set of causes.

Thermal runaway. If the hotend temperature drops unexpectedly — a failing heater cartridge, a loose thermistor, or a draught hitting the heater block — modern firmware detects it and stops the print. You will see a "thermal runaway" error on screen. Do not disable this safety feature; it protects against fires. Check that the thermistor is seated firmly in the block, and test the heater cartridge with a multimeter. A typical 40 W cartridge reads around 10–20 ohms. Open circuit means it has failed.

Filament runout sensor. These occasionally false-trigger even when there is plenty of filament on the spool. Disable it temporarily to confirm. If prints complete with the sensor bypassed, the sensor itself — usually a dirty or misaligned optical switch — is the problem.

A tangle on the spool. The filament gets crossed over itself during winding, pulls tight, and the extruder cannot pull it through. Keep the loose end under control when loading a new spool and never let it spring back across the wound layers.

Slicer and settings problems

If nothing mechanical has changed but prints are suddenly bad, check your slicer profile. The most common settings problems are:

• Print temperature too low — layers do not bond and the surface looks rough or crumbly.
• Print speed too high for the temperature — the hotend cannot melt filament fast enough and the extruder starves.
• Retraction too aggressive — on Bowden setups in particular, high retraction pulls molten filament up into the heatbreak and causes clogs.
• Wrong nozzle diameter in the profile — a profile set for 0.4 mm running on a 0.6 mm nozzle will massively under-extrude.

Find a community profile for your exact printer and material and compare values against your current settings. A single number out of range can ruin an otherwise fine setup.

Mechanical checks

If everything above looks right, do a quick mechanical inspection.

Belts stretch over time. A loose X or Y belt causes shifted or sloppy layers. Pluck each belt like a guitar string — it should feel taut. If it feels slack or the layers are shifting under fast direction changes, tension it until it gives a firm, low note.

On printers that use V-slot wheels, check for wobble by pushing the printhead gently with the machine powered off. Any play means the eccentric nuts need adjustment — tighten until the play disappears but the carriage still rolls freely.

A dirty or bent Z-axis lead screw causes regular horizontal banding across the surface. Clean the screw with a dry cloth and apply a thin coat of PTFE-based grease.

When to mail it in

Some faults are quick to diagnose in person and slow to diagnose remotely. A failed heater cartridge, a cracked thermistor, a stripped extruder gear, or a damaged mainboard are all hands-on repairs — they need a multimeter and the right spare parts. We work on all common FDM printers here, and we will tell you honestly if a repair is not worth the cost against buying a replacement part you can fit yourself. Send a short description of what your printer is doing to our contact page and we will give you a realistic picture of what is involved before you commit to posting anything.