i realize this is an OLD thread but as I was on a quest to identify this code I will post up what i found in case others need it.
A P0603 is a loss of Keep Alive Memory (KAM) which can either be cleared with a scan tool or by removing battery power from the PCM KAM power circuit that supplies 12-volts to maintain the memory. Clearing KAM with a scan tool will not trigger a P603 but if battery power is low or removed altogether the memory is lost and the code is set. This could be caused by a module disconnect, programming, low battery, disconnected battery or a wiring concern that causes KAM power to be removed. Like most codes, a P0603 is cleared after 40 engine warm-up cycles without the fault being detected.
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