استبدال وحدة تحكم المحرك أو إعادة برمجتها

Not a fault at all, but the loss of everything the module had learned about its own vehicle. A modern engine module accumulates knowledge that no factory could have supplied: injector calibration codes entered per cylinder, injection pump timing and quantity corrections, throttle and pedal end stops, fuel trims built up over thousands of miles, misfire adaptation for this particular crankshaft's manufacturing runout, transmission adaptations, and the vehicle's own identity and immobiliser pairing.

Replacing or reflashing the module discards all of it, and the vehicle then runs on defaults. Defaults feel like wear: hard starting, a harsher note, smoke, uneven idle, poor economy, hesitation. So a car that runs badly immediately after module work presents as a faulty module or a faulty new part, and the natural next step is to replace something else — which changes nothing, because the missing item is data rather than hardware.

The rule worth carrying: a vehicle running badly straight after module work should have its adaptations and calibrations checked before any component is suspected. Restoring them is part of the job, takes minutes with the right equipment, and is the single most common thing left undone.

It is also why disconnecting a battery is not always free. On some vehicles adaptations survive in non-volatile memory; on others they do not, and the car drives differently for a week afterwards while it relearns.