All about the Check Digit code - Calculating
The check digit is a single digit in the 9th place of the 17th character VIN which began starting 1981. The check digit number was used to determine and verify that the accuracy of the VIN.

The check digit appears on any transfer documents containing the VIN which is prepared by the manufacturer and given to the first owner for purposed other than resale. Thus, the VIN's of any 2 vehicle manufactured within a 30 year period would never be identical.

After all the 17 digits of the VIN has been determined by the manufacturer, the check digit is calculated by carrying out a mathematical computation specific below. This is based on VIN position, sample VIN, assigned value code, weight factor and multiplying assigned value times the weight factors. The value are then added and the total is divided by 11. The remainder of this would be 0 - 9 (unless if 10, then the letter "X" would be designated as the check digit code.
Below You will find the calculation used to verify the accuracy to your VIN (1981-present)
Third Generation: 1982 - 1992
Check Digit Calculation
1. Any NUMBERS in VIN the retain their original value (not replace).
0 = 0 2 = 2 4 = 4 6 = 6 9 = 9
1 = 1 3 = 3 5 = 5 8 = 8  
2. Each LETTER in the VIN is assigned a numerical value.
A = 1 D = 4 G = 7 K = 2 N = 5 R = 9 U = 4 X = 7
B = 2 E = 5 H = 8 L = 3 O = 6 S = 2 V = 5 Y = 8
C = 3 F = 6 J = 1 M = 4 P = 7 T = 3 W = 6 Z = 9
3. The assigned value of each VIN character (letter/number) is then multiples by the position weight factor.
Position Weight Factor
1st = 8 4th = 5 7th = 2 10th = 9 13th = 6 16th = 3
2nd = 7 5th = 4 8th = 10 11th = 8 14th = 5 17th = 2
3rd = 6 6th = 3 9th = 0 12th = 7 15th = 4  
4. The resulting products are then added, and that total is then divided by 11.
5. The numerical remainder is the check digit. If the remainder happens to be 10, the result is "X" as being used for the check digit.
6. Your VIN is CORRECT if your remainder (which we now know is the Check Digit code) is the same as the 9th digit in your VIN.
Below is a visual calculation of a SAMPLE VIN. (Check Digit in RED)
VIN 1   G   1   F   P   2   3   8   5   N   L   1   2   7   9   2   6
Assigned Values 1   7   1   6   7   2   3   8   5   5   3   1   2   7   9   2   6
Weight Factor 8   7   6   5   4   3   2   10   0   9   8   7   6   5   4   3   2
Add the products (1 x 8)   (7 x 7) + (1 x 6)   (6 x 5)   (7 x 4)   (2 x 3)   (3 x 2)   (8 x 10)   (5 x 0)   (5 x 9)   (3 x 8)   (1 x 7)   (2 x 6)   (7 x 5)   (9 x 4)   (2 x 3)   (6 x 2)
Product Totals 8 + 49   6 + 30 + 28 + 6 + 6 + 80 + 0 + 45 + 24 + 7 + 12 + 35 + 36 + 6 + 12
Sub Total

390
DIVIDE by 11

35 w/ remainder 5

5 is the same as the 9th digit (Check Digit) so the VIN IS correct


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