خلل في برمجية وحدة التحكم

A defect in a module's own program or calibration rather than in anything it is wired to.

This is the cause people reach for when nothing else fits, and it is right far less often than it is chosen — so it is worth knowing what genuine software faults look like, because they have a signature.

They are reproducible and specific. A real software defect does the same wrong thing under the same conditions on every vehicle of that build: a fault that appears only above a certain speed, only when a particular combination of options is fitted, or only after a specific sequence of events. A fault that comes and goes with vibration, weather or temperature is wiring, however unlikely the wiring seems.

They also have paperwork. A manufacturer that knows about a software defect publishes a bulletin and an updated calibration, so the first genuinely useful step is to ask whether an update exists for this module on this vehicle — which is a search rather than a diagnosis, and it is free.

Corruption is the other half of this cause, and it usually has a story. A flash interrupted by a voltage drop leaves a module part-programmed; so does a battery going flat mid-update or a charger with poor regulation. This is why battery support units are standard practice during programming and why "it started after the car was at the dealer" is a real clue rather than a coincidence.

One caution about the repair. Reprogramming a module clears its learned adaptations — fuel trims, throttle positions, transmission clutch fills — so a car can drive noticeably worse for a while afterwards while it relearns. That is expected, and it is not evidence the update failed.