A VIN is valid when it is exactly 17 characters, uses only the legal alphabet (no I, O, Q), and its 9th character equals the mod-11 check digit of the other 16. Miss any one of those and it is invalid. Below, a valid VIN and the same VIN broken three ways, with the exact reason each fails.
| Check | Rule | Common failure |
|---|---|---|
| Length | Exactly 17 characters | A dropped or extra character (16 or 18) |
| Alphabet | Only A–H, J–N, P, R–Z, 0–9 | An I, O or Q typed for 1 or 0 |
| Check digit | Position 9 = mod-11 checksum | Any single mistyped character |
The check digit is what makes VIN validation strong: because position 9 is computed from every other character, a single transposed or wrong character breaks it — which is exactly the point. If a VIN fails only the check-digit test, re-read it carefully; most failures are one mistyped character. See the check-digit calculation for the math.
Confirm it is 17 characters, contains no I/O/Q, and that position 9 matches the mod-11 check digit of the other 16 characters. All three must hold. The decoder runs these checks in your browser.
Almost always a typo. Because position 9 is a checksum, one wrong character makes it fail. Also check you did not type an I, O or Q — those never appear in a VIN.
A modern VIN is always 17 characters. Vehicles before 1981 used shorter, non-standard VINs, but any 1981-or-newer VIN that is not 17 characters is invalid.