When the left needle thread breaks it's tempting to just re-thread that path alone - but this almost never fixes it. A full re-thread of all elements is nearly always the solution.

• Re-thread all elements completely from scratch, in the correct order - loopers first, then needles
• Before threading, turn the handwheel towards you until the needles are at their highest position - this puts the loopers in the correct position to accept the threads properly
• Raise the presser foot before threading so the tension discs open correctly
• Go slowly and refer to your manual rather than going from memory

This is the single most specific and common cause of left needle thread breaking. It's easy to do wrong and easy to miss when checking.

• When threading the lower looper, the thread must pass under the upper looper - not over it
• If it passes over the upper looper, the left needle thread gets trapped as the stitch forms and snaps repeatedly
• After re-threading, visually check this point before sewing - it only takes a second

An incorrect stitch finger position can cause the left needle thread to snap as stitches form.

• Make sure the stitch finger is set to N for normal overlocking
• If it's been left in the R (rolled hem) position from a previous project, switch it back to N before continuing

A bent, blunt or incorrectly fitted left needle is a common cause of thread breaking on that path specifically.

• Replace the left needle with a fresh one of the correct type for your overlocker
• Make sure it's fully inserted with the flat side to the back and tightened securely

Left needle tension that's too tight will cause the thread to snap under the strain of sewing.

• Ease the left needle tension dial back slightly and test on scrap fabric
• Make small adjustments and test after each one rather than making large changes

Pulling or pushing the fabric while sewing puts stress on the needle threads and can cause the left needle thread to snap.

• Let the feed dogs move the fabric at their own pace
• Guide the fabric gently with your hands but never tug it through the machine

Sewing too fast can cause the left needle thread to snap, particularly on thicker fabrics or multiple layers.

• Slow down and let the machine work at a steady, comfortable pace

Once you've worked through the steps, test on scrap fabric of a similar weight to your project. Run a decent length of stitching and confirm the left needle thread is holding before returning to your work.