مفتاح الخفض القسري معيب

The full-throttle downshift request. On older automatics it is a physical switch under the accelerator's last centimetre of travel; on newer cars it is a firm detent at the end of the pedal sensor's range that behaves as one. Either way it corrodes, sticks and falls out of adjustment like any switch that lives on the floor of a car — and the failure only shows at full throttle, which is why it survives every normal test drive.

Check the cheap explanations first: a floor mat riding up under the pedal, a switch knocked out of adjustment by footwell work, a corroded connector. Where it is a separate part it is inexpensive; on pedal-sensor cars the repair is usually the pedal assembly.