Rule out the usual hiding places first. If the piece really is gone, check the set piece by piece
and leave with the element ID LEGO asks for, or a link to buy the part.
Bar 1L with Clip [Cut Edges and One Side Hole] [Gap in Clip]BlackPart 48729bElement 6278152 or 60373471 spare in the box×1
Technic Connectors
1 piece
Technic Axle Connector 2 x 3 with Ball Socket, Open Lower Axle HolesBlackPart 93571Element 4610948×1
Tick each piece as you find it, or mark it missing.
When a piece really is missing
Ask LEGO
LEGO takes requests for pieces missing from a new set through its customer service site. The form asks for
the set number and each piece's element ID, the number printed beside the piece in the parts list at the back
of the instructions. This checker lists both.
Mark a piece missing and your list links it to an eBay search for that part, which also covers pieces from
older or second-hand sets.
Keep a record
Copy or download your missing list (the CSV opens in any spreadsheet), or print the whole checklist and tick
pieces off on paper. Your marks stay in this browser, so you can check a big set over several sittings.