Normal Power Down Sequence

WARNING: This page is about a different car, the 2006 Mercury Mariner and 2006 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

The powertrain control module (PCM) must conduct a normal power-down sequence. Whenever the key is turned to the OFF or ACC position, modules that are powered up by the RUN circuit immediately shut down. However the PCM, transaxle control module (TCM), and the traction battery control module (TBCM) stay on, until the power-down sequence is complete. The PCM keeps the TCM alive by controlling the power sustain relay (PSR) which provides power to the TCM. The TBCM is powered directly from the low voltage battery which permits wake-up function when the vehicle is off. During the power-down sequence the PCM:

cuts the power to injectors and ignition coils (engine shut down).
requests the TCM to disable high voltage inverters.
disables the DC/DC converter.
requests the TBCM to open the high voltage contactors.
requests the TCM to discharge the high voltage inverter capacitors.
opens the power sustain relay.

If the power down sequence does not execute correctly, it is considered an abnormal shut-down, which may result in the PCM, the TCM and the TBCM storing diagnostic trouble codes (DTCs).