Avarie interne de la boîte de vitesses

Mechanical failure inside the transmission — a clutch pack, a planetary set, a bearing or a shaft — as opposed to a control, sensor or hydraulic fault outside it. It is the conclusion of last resort and belongs there, because it is the most expensive answer available and almost everything cheaper produces the same codes.

Ratio codes are what it usually announces itself with, since the module compares input and output speeds and finds a ratio the gearbox should not be able to produce. But a solenoid, low fluid, a stuck valve or a worn valve body will all give the same reading. Rule those out first, in that order — they are cheaper, commoner, and each is a genuine repair when found.

The evidence that distinguishes it is physical. Debris in the pan and on the magnets — clutch material, metal, or bronze from a bushing — is the strongest indicator available, and burnt-smelling fluid supports it. Drop the pan before quoting, because it converts a suspicion into a finding. A gearbox slipping in one gear and correct in others points at a specific clutch pack rather than at general wear, which is worth knowing before deciding between repair and replacement.