What is causing reoccurring p-0603 code?

Question

Could it be audio system w/3 amps, after market Optima yellow top battery w/isolator and secondary audio battery?

Answer 1

P0603 ECM power supply

If you have a open circuit or short between the ECM and the battery this would set this code condition and if Fuse IPDM which is a 20 amp fuse is open then this would set the code condtition you have . Just test for power IPDM to the ECM for power with key off engine off