When the ECU is turned on (or is recovering from a powered reset), the bootloader will enter reprogramming mode if the FEPS pin is asserted, if there is an invalid application image in memory, or for certain targets if the ECU is running the application and allowing reprogramming while a reprogramming request is received (FEPS-less).

In reprogramming mode, the ECU listens to the communications buses for instructions to reprogram, as described in Section 4.5, “Programming an ECU”.