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The code P0603 is in ost case set due to the performance chip.
If equipped, remove and recheck operation. If the vehicle is not equipped with a chip, or removing it makes no difference, monitor the voltage on fuse #22 in the interior fuse panel to see if the battery voltage goes away when the problem occurs. If the voltage on fuse #22 is dropping or going away, check for a loose terminal connection on the PCM power relay. Also try swapping the PCM power relay with a similar part to eliminate a faulty relay. If the relay is OK, but the voltage on fuse 22 is dropping, suspect an internal problem with the interior fuse panel.