Layer shift diagnosis

Layer shift is when the print suddenly steps sideways (X or Y) mid-build and continues printing offset from the rest. Distinguishing it from layer adhesion failure: a layer shift creates a clean step. A delamination creates a torn seam.

The three causes

In order of frequency:

1. Mechanical: belts, pulleys, or rails
2. Thermal: stepper drivers overheating
3. Software: corrupt gcode or slicer bug

1. Check belts and pulleys

Push on the belts. They should be guitar-string tight but not bowstring tight. If you can deflect a 100mm span more than ~3mm with light finger pressure, they're loose.

Check the pulley grub screws. Spin the motor by hand -- if the pulley slips on the shaft, the grub screw is loose. This is the #1 cause of intermittent layer shift on Ender / Prusa printers.

For Bambu and other Core-XY printers, both motor pulleys matter. Either can slip.

2. Listen for skipped steps

Watch and listen during a print:

• Click-clack noise from a corner: the head crashed into something (loose part, stuck infill, support that pulled up). Print recovers but the head has lost steps.
• Whirring/squealing on Y travels: motor underpowered or driver overheating
• Silence then shift: thermal shutdown of a stepper driver

Print the same model on the bed in 3 different orientations. If shifts always happen at the same Z, it's the model. If they happen at random Zs, it's mechanical.

3. Check for obstructions

Pull the spool off the holder and turn it freely by hand. If it binds or catches, the spool is mis-spooled. Tangled spools yank the extruder and cause shifts on direct-drive printers.

Look at the cable chain on Y -- a worn drag chain can snag on the gantry edge.

4. Stepper drivers

If you're losing steps under heavy load (high accelerations, dense infill), the stepper drivers may be cutting current to protect themselves. Symptoms:

• Shifts happen on big infill blocks
• Shifts disappear when you reduce print speed by 30%
• Shifts repeat at the same time-into-print on long jobs (thermal buildup)

Lower print speed and acceleration as a test. If shifts stop, the driver is overheated. Add a cooling fan over the mainboard or upgrade to TMC2209 / TMC5160 drivers.

5. Gcode / slicer

Rare but it happens. If a slicer update suddenly causes shifts on prints that previously worked:

• Re-slice the model with the previous slicer version
• Check slicer release notes for known bugs
• Save the gcode and inspect with gcodeviewer -- look for absolute moves to weird coordinates

Fixes by symptom

When it's the print itself

Sometimes a tall thin print tips over slightly mid-build, the head clips the top, loses steps, and now everything above is offset. The fix is "print with a brim and support" not "tune the driver."