Since the late 1990s, most vehicles have used a transponder chip embedded in the key that communicates with an immobilizer system in the car — if the chip's code doesn't match what the car expects, the engine simply won't start, even with the correct-looking key in the ignition. This is the part of key replacement that a hardware store key-cutting kiosk can never handle, because it requires actually talking to the vehicle's electronics.
We program transponder keys using OBD-II diagnostic connections for most modern vehicles, and proprietary programming methods for older or manufacturer-specific systems where OBD-II access isn't sufficient. This is almost always the missing step when a 'correctly cut' replacement key still won't start the car.
