Vanne de dérivation du refroidisseur de boîte défectueuse

The valve that routes transmission fluid either through its cooler or around it, faulty so the routing is stuck. The bypass exists because cold fluid is as much a problem as hot fluid — thick cold fluid shifts badly and wastes fuel, so the valve keeps it out of the cooler until it warms.

Stuck in bypass, the fluid never cools, which is the damaging direction: sustained high temperature oxidises the fluid, glazes the clutch material and shortens the transmission's life considerably. Stuck in the cooled position, the fluid never warms properly, so cold shift quality is poor and the module's temperature compensation works against reality.

The complaint is often nothing at all until the fluid is cooked, which is why the code matters more than the symptoms. Compare fluid temperature behaviour across a warm-up and a sustained run — fluid that climbs and keeps climbing is not being cooled, and fluid that never reaches its operating window is being cooled too soon. Check the cooler itself is not blocked while investigating, since a restricted cooler produces the same overheating with a perfectly good valve.