Batterie débranchée ou calculateur reprogrammé

The vehicle lost power or had a module rewritten, and the learned values that lived in volatile or reset-on-write memory went with it.

A modern vehicle knows a great deal about itself that no factory supplied, and most of it is learned rather than configured: window motor end stops, sunroof panel limits, throttle and pedal positions, steering angle zero, fuel trims, transmission adaptations, seat and mirror memory, radio codes and immobiliser pairings. A battery disconnection can erase any of it, and a reflash erases whatever it overwrites.

The consequence is a vehicle that behaves worse after a job than before it, which reliably gets blamed on the work or the new part. The classic examples are worth recognising because each has a free fix:

• A window that will only inch up while the switch is held has lost its normalisation — the anti-trap protection cannot be trusted without it, so one-touch is disabled deliberately. Re-initialising takes seconds.
• A sunroof that refuses to move has lost its position memory and needs its reset sequence.
• An engine that idles roughly or hesitates for a few days is relearning its fuel and idle adaptations, and usually settles by itself.
• A gearbox shifting harshly is doing the same, which is why a road cycle is part of the job rather than optional.

So the first question after any complaint that began with battery or module work is not what broke, but what needs relearning. Checking that costs minutes and resolves a large share of these outright.

It is also the reason a memory-saver is worth using during battery work, and why disconnecting a battery is never quite free on a modern vehicle.