PO603 means you probably have a tuner and you changed programming, When you do this it cycles power to the PCM.
Also uppdate your signature so others know what vehicle you may be asking about
Here are some code definitions
1999 Powertrain Control System Electronics Diagnostic Guide - 1994-1997 Power Stroke FAQ
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Ken
2002 F-350 CCLB 4X4, HD4R100 , Riffraff FRx, Riffraff CAC Boots & PRI's, Bellowed Up-Pipes, 4" Diamond Eye exhaust, Powerslot rotors, ATS Ported Compressor Housing, DP Tuner-F6, Autoenginuity, EEC-V Breakout Box, KCM S.S. HPX Line, S&B Filters intake, Quad Autometer gauge pod, Inovations canopy, Banks Big Head wastegate actuator, 6.0L Trans Cooler, Spin-on Transmission filter with custom bracket, Zoo-dad, Dieselsite Coolant Filter, Cobra 29LTD/NW CB, DVD and always looking to do more....  
Powerstroke Acronyms
TSB Information
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