Conditions for Setting the DTC: P0603
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
The ECM check sum value at the last power down does not match the ECM check sum value during the current power up for one sample. Each sample is 10 milliseconds. The ECM detects that the check sum value at the last power down does not match the ECM check sum value during the current power up is incorrect for 0.01 second. This diagnostic runs once per ignition cycle at power up.