P0603

P0603 indicates the Keep Alive Memory (KAM) section of the PCM has failed its self-test, typically because the constant-power feed to KAM was lost or because the memory cells will no longer retain learned data (idle adapts, fuel trims, transmission shift adapts, immobilizer counters). The cheapest first move is to check the battery: a fully dead battery, jumpstart event, battery disconnect lasting more than a few minutes, or a corroded battery terminal will all reset KAM and on many platforms set a one-time P0603. Clear the code, drive normally to relearn fuel trims and idle, and see if it returns. Verify the KAM/B+ fuse (often labeled BATT, ECM-B, or PCM-B in the underhood fuse box) has constant 12V with key off, and check for parasitic draw issues that could be cycling that feed. Inspect the battery cable terminations and the PCM main-power circuit for high resistance — anything over 0.1V drop on the B+ feed to the PCM under load is suspect. The expensive misdiagnosis is replacing the PCM for a single P0603 that set after a known battery disconnect; on most domestics built after 2005, P0603 will self-clear after a few drive cycles and never return if the underlying power-supply issue is resolved.

Vehicle-specific patterns: 2004-2010 Ford F-150, Explorer, and Escape commonly set P0603 after a dead battery or jump-start, addressed by simply clearing the code and completing the KAM relearn drive cycle per Ford TSB 06-26-3. 2003-2007 GM trucks with corroded positive battery terminals (the side-post style is especially prone) throw P0603 intermittently until the cable ends are cleaned or replaced. 2005-2011 Chrysler/Dodge Ram with the TIPM lose KAM power when the internal TIPM B+ relay degrades, setting P0603 along with random no-starts, covered by FCA TSB 08-031-15. 2006-2012 Honda Accord and Civic set P0603 after aftermarket alarm or stereo installs that briefly disconnect the constant 12V backup to the PCM. Estimated repair: $0 to $1,200.