What the ninth character does
The check digit is calculated from the other VIN characters using assigned transliteration values and position weights. The resulting remainder determines the ninth character. If the remainder is 10, the check digit is represented by X.
What it does not do
A matching check digit does not prove that a vehicle is legitimate, accident-free, correctly titled or free of fraud. It is primarily a structural validation mechanism.
Why a VIN may still fail to decode
A VIN can be mistyped, belong to an older or unusual vehicle, contain manufacturer coding that is incomplete in available data, or be outside the standardized 17-character format.