Reference
How the check digit works
Position 9 is not part of the vehicle description. It is a checksum, computed from the other sixteen characters, that catches most transcription errors.
The three steps
Defined in 49 CFR 565. Every character is converted to a number, multiplied by a positional weight, and the total is reduced modulo 11.
1. Transliterate
Digits keep their value. Letters map to numbers as follows — note that I, O and Q are absent because they never appear in a VIN:
| Letter | Value |
|---|---|
| A J | 1 |
| B K S | 2 |
| C L T | 3 |
| D M U | 4 |
| E N V | 5 |
| F W | 6 |
| G P X | 7 |
| H Y | 8 |
| R Z | 9 |
2. Apply positional weights
Each position carries a fixed weight. Position 9 — the check digit itself — carries weight 0, so it does not contribute to the sum that produces it.
8 7 6 5 4 3 2 10 0 9 8 7 6 5 4 3 2
3. Divide by 11
Sum every value multiplied by its weight, then take the remainder modulo 11. That remainder is the check digit. A remainder of 10 is written as the letter X — the only place a letter appears in position 9.
Why it fails
The usual reason is a typo. Modulo 11 catches every single-character substitution and most transpositions, which is exactly the error people make copying a VIN off a windshield.
The other reason is legitimate: the check digit is mandatory for vehicles built for the North American market, but not elsewhere. A vehicle imported from Europe or Japan may carry a genuine VIN that fails this test.