This is one of the most frequently asked questions in BMW tuning, and it deserves an honest answer.
BMW dealers can detect ECU modifications. Modern BMW diagnostic systems (ISTA) read the DME software version and compare it against the expected factory version for your VIN. If the calibration data differs from the factory reference, it will be flagged. Some dealers check proactively; others only check when investigating a warranty claim.
How this affects warranty depends on jurisdiction and the specific claim. In most European markets, the Magnuson-Moss equivalent principle means that the manufacturer must demonstrate that the modification caused the failure to deny a warranty claim. A tuned engine with a failed water pump, for example, would be difficult to deny since the modification didn't cause the failure.
However, claims for engine internals, turbocharger or transmission on a tuned vehicle are likely to be scrutinised more closely. Consider this carefully before tuning a vehicle under warranty.
JB Remap provides stock restoration files for all calibrations, allowing you to revert before a service visit.