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LEGO Missing Piece Checker

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.

Computer Science & AI Kit 3-5

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45521 Computer Science & AI Kit 3-5

309 pieces in 85 part and color combinations, plus 4 minifigures.

The official piece count includes the 12 parts inside the set's minifigures, which this inventory lists separately. Spare parts are counted in neither figure.

Before you call it missing

Builders who report a missing piece often find it in one of these places. Tick each one off as you look.

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Piece checklist

Bricks 48 pieces

  • Brick 2 x 4 Blue Part 3001 Element 300173 or 300123 ×2
  • Brick 2 x 4 Bright Light Orange Part 3001 Element 6100027 or 4645053 ×2
  • Brick 2 x 2 Coral Part 3003 Element 6422920 ×4
  • Brick 2 x 2 Dark Purple Part 3003 Element 4653960 or 4589613 ×4
  • Brick 2 x 4 Dark Purple Part 3001 Element 4626935 or 4589602 ×4
  • Brick 2 x 6 Dark Purple Part 2456 Element 6147004 or 4589615 ×4
  • Brick 2 x 2 Dark Turquoise Part 3003 Element 6314091 or 4120399 ×4
  • Brick 2 x 4 Dark Turquoise Part 3001 Element 6249422 or 6235115 ×4
  • Brick 2 x 4 Green Part 3001 Element 4106356 or 300128 ×2
  • Brick 2 x 4 Medium Lavender Part 3001 Element 6013583 or 4655173 ×4
  • Brick 2 x 4 Red Part 3001 Element 300121 ×2
  • Brick 1 x 2 x 5 without Side Supports Trans-Purple Part 46212 Element 6514089 or 6350502 ×4
  • Brick 2 x 2 White Part 3003 Element 6552095 or 6548256 ×4
  • Brick 2 x 8 White Part 3007 Element 6619309 or 6033776 ×4

Plates Special 38 pieces

  • Plate Special 2 x 2 with Rubber Foot Black Part 109373 Element 6492701 ×4
  • Plate Special 1 x 4 x 2/3 with 1 x 2 Bottom Cutout Coral Part 7769 Element 6598033 ×4
  • Bracket 1 x 2 - 2 x 2 Dark Purple Part 44728 Element 6334093 or 4277936 ×6
  • Plate Special 1 x 4 with 2 Studs with Groove [New Underside] Dark Purple Part 41740 Element 6253191 ×4
  • Bracket 1 x 2 - 2 x 2 Inverted Dark Turquoise Part 99207 Element 6513766 ×4
  • Plate Special 2 x 3 with Cloud Edge Dark Turquoise Part 5518 Element 6544526 ×4
  • Plate Special 2 x 2 x 2/3 with Axle Hole and Grill on Sides (Haybale) Light Bluish Gray Part 71752 Element 6609654 ×6
  • Plate Special 2 x 4 with Pins, Reinforced Light Bluish Gray Part 40687 Element 6408542 ×2
  • Plate Special 1 x 2 with Three Teeth [Tri-Tooth] White Part 15208 Element 6046381 ×4

Bricks Sloped 22 pieces

  • Brick Sloped Inverted 33° 3 x 1 with Internal Stopper and No Front Stud Connection Black Part 4287c Element 428726 ×4
  • Brick Sloped Inverted 45° 2 x 1 Coral Part 3665 Element 6467785 ×4
  • Brick Sloped 33° 3 x 2 Dark Purple Part 3298 Element 6055875 or 4224864 ×4
  • Brick Sloped 45° 2 x 1 with Bottom Pin Trans-Purple Part 3040b Element 6609649 or 3040126 ×6
  • Brick Sloped 33° 3 x 1 White Part 4286 Element 6632142 or 428601 ×4

Plates Round Curved and Dishes 22 pieces

  • Plate Round Corners 8 x 8 x 2/3 Circle with Reduced Knobs Dark Purple Part 66790 Element 6326551 ×3
  • Plate Round Corner 3 x 3 with 2 x 2 Round Cutout Dark Turquoise Part 68568 Element 6323161 ×4
  • Plate Round Corners 8 x 16 x 2/3 Circle with Reduced Knobs Dark Turquoise Part 5251 Element 6482578 ×1
  • Plate Round 2 x 2 with Rounded Bottom [Boat Stud] Light Bluish Gray Part 2654 Element 4278273 or 4276273 ×6
  • Plate Round 4 x 4 with Pin Hole White Part 60474 Element 6564904 or 4515347 ×4
  • Plate Round Corner 3 x 3 with 2 x 2 Round Cutout White Part 68568 Element 6552185 or 6339103 ×4

Technic Bricks 22 pieces

  • Technic Brick 1 x 6 [5 Pin Holes] Black Part 3894 Element 389426 ×4
  • Technic Brick 1 x 2 with Axle Hole Type 1 [+ Opening] and Bottom Pin Coral Part 32064a Element 6439447 ×4
  • Technic Brick 1 x 2 [1 Pin Hole] Dark Turquoise Part 3700 Element 6345402 or 4140338 ×4
  • Technic Brick 1 x 4 [3 Pin Holes] White Part 3701 Element 6601913 or 370101 ×4
  • Technic Brick 1 x 8 [7 Pin Holes] White Part 3702 Element 4582543 or 370201 ×4
  • Technic Brick 5 x 6 Right Angle Double White Part 7469 Element 6548852 ×2

Bricks Special 18 pieces

  • Brick Special 2 x 2 with Pin and Axle Hole Black Part 6232 Element 6365577 or 6070692 ×4
  • Brick Special 2 x 4 with 3 Axle Holes Black Part 39789 Element 6244917 ×4
  • Brick Special 2 x 4, with 2 x 2 Brick with Top Pin and 1 x 2 Side Plates Light Bluish Gray Part 30592 Element 6301425 or 6278504 ×2
  • Brick Special 1 x 2 with Pin - Bottom Stud Holder White Part 44865 Element 6406522 ×4
  • Technic Brick 2 x 4 with 4 Pin Holes, 2 Pins White Part 112174 Element 6598035 ×4

Bricks Round and Cones 16 pieces

  • Brick Round 2 x 2 with Axle Hole Trans-Clear Part 3941 Element 6514281 or 6273152 ×4
  • Brick Round 2 x 2 D. Tube with 45° Elbow and Axle Holes (Crossholes) at Each End [LONG] Trans-Purple Part 5489 Element 6609651 ×4
  • Cone 1 x 1 [Top Groove] Trans-Purple Part 59900 Element 6514023 or 6337603 ×8

Plates 16 pieces

  • Plate 2 x 4 Coral Part 3020 Element 6305455 ×4
  • Plate 2 x 8 Dark Purple Part 3034 Element 6109931 ×4
  • Plate 2 x 6 Dark Turquoise Part 3795 Element 6259794 or 4121503 ×4
  • Plate 2 x 12 White Part 2445 Element 6552091 or 244501 ×4

Technic Axles 16 pieces

  • Technic Axle 4 Black Part 3705 Element 370526 ×2
  • Technic Axle 5 Light Bluish Gray Part 32073 Element 32073194 or 4211639 ×2
  • Technic Axle 2 Notched Red Part 32062 Element 4142865 ×4
  • Technic Axle 6 Red Part 3706 Element 6130002 or 4191526 ×2
  • Technic Axle 3 Yellow Part 4519 Element 6130007 ×6

Tiles Round and Curved 16 pieces

  • Tile Round 2 x 4 Black Part 66857 Element 6331483 ×4
  • Tile Round 2 x 2 with Black Circle, White Circles / Eye Print White Part 14769pr9967 Element 6522844 ×12

Technic Gears 12 pieces

  • Technic Gear 12 Tooth Double Bevel Black Part 32270 Element 4177431 ×2
  • Technic Gear 36 Tooth Double Bevel Black Part 32498 Element 4255563 or 4177434 ×2
  • Gear Track Straight 3 x 16 Dark Purple Part 7333 Element 6598043 ×2
  • Technic Gear 20 Tooth Double Bevel with Axle Hole Type 1 [+ Opening] Tan Part 18575 Element 6346517 or 6084724 ×2
  • Gear Box 4 x 4 x 2 for Gear Track Trans-Clear Part 7332 Element 6598042 ×2
  • Gear Track Curve 90° 17 1/2 x 17 1/2 White Part 7334 Element 6598044 ×2

Technic Pins 12 pieces

  • Technic Pin without Friction Ridges Lengthwise Yellow Part 3673 Element 6331822 ×12

Energy Effects 8 pieces

  • Wave / Flame Rounded with Base Rim Trans-Light Blue Part 6126b Element 6167739 ×4
  • Wave / Flame Rounded with Base Rim Trans-Orange Part 6126b Element 6167741 or 4261397 ×4

Plates Wedged 6 pieces

  • Wedge Plate 4 x 6 Cut Corners Black Part 32059 Element 4129572 ×2
  • Wedge Plate 7 x 6 Boat Bow with Stud Notches White Part 50303 Element 6609645 or 4239366 ×4

Bricks Curved 4 pieces

  • Brick Curved 6 x 1 Black Part 42022 Element 4160393 ×4

Plants & Trees 4 pieces

  • Plant, Leaf 2 x 6 x 1 with 2 Studs Trans-Clear Part 3565 Element 6548957 ×4

Technic Connectors 4 pieces

  • Technic Axle Connector Smooth [with x Hole + Orientation] Coral Part 59443 Element 6544409 ×4

Tiles 4 pieces

  • Tile 1 x 6 with Groove Coral Part 6636 Element 6425367 or 6258096 ×4

Tiles Special 4 pieces

  • Tile Special Round 2 x 2 with Hole Dark Turquoise Part 15535 Element 6491435 ×4

Tubes and Hoses 4 pieces

  • Axle Hose, Soft 7L Black Part 32580 Element 6132744 ×4

Wheels and Tyres 4 pieces

  • Wheel Smooth 24 x 14mm with Pin Hole Black Part 5253 Element 6482581 ×2
  • Wheel with 4 Spokes with 2 Holes, 4 Inserts and Black Integral Tire Black Part 112175pat0001 Element 6598038 ×2

Electronics 3 pieces

  • Motor 10 x 6 x 3, Science White Part 118091 Element 6669513 or 6610998 ×1
  • Sensor, Colour, Science White Part 118092 Element 6666299 or 6611005 ×1
  • USB Cable White Part 114253 Element 6554054 ×1

String, Bands and Reels 2 pieces

  • Technic Rubber Belt Medium [24mm ID] [Round Cross Section] Red Part 85544 Element 4544145 or 4544143 ×2

Technic Special 2 pieces

  • Pulley, 2-Stage Black Part 7335 Element 6598041 ×2

Gear Parts 1 piece

  • Activity Card ? for Science Kits [No Color/Any Color] Part 119190 Element 6633001 ×1

Tools 1 piece

  • Brick and Axle Separator v2.0 Dark Turquoise Part 96874 Element 6254100 ×1

Minifigures 4 figures

  • Monochrome Bright Green Minifigure, checked as a whole figure ×1
  • Monochrome Red, 28621 Head Minifigure, checked as a whole figure ×1
  • Monochrome Blue Minifigure, checked as a whole figure ×1
  • Monochrome Bright Light Orange, 28621 Head Minifigure, checked as a whole figure ×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.

Request a replacement from LEGO

Buy the piece

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.

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