It was kind of slow today. Here is what I extracted concerning the P0603 and possible causes and solutions.

Hope it helps.

MA1 CONTINUOUS DTC P0603 IS SET WHEN THE PCM HAS LOST KAPWR
Note: This code may have been caused by disconnecting the PCM or the batteries.

Possible causes:
Battery or PCM has been disconnected.
Battery cables loose or poor connection.
Battery discharged.
Open in KAPWR circuit.
Blown fuse.
PCM.

Check for other codes.
Key on, engine off.
Clear DTCs.
Cycle key off, then on.
Perform retrieve Continuous DTCs.

MA2 CHECK 5 AMP FUSE (#2 FOR F-SERIES)
Key off.
Remove and inspect fuse 2 (F-Series) If fuse is blown, replace it.
Measure voltage between fuse B+ terminal and ground.

Is Voltage greater than 10 VDC?

Yes: go to "MA3"

No: REPAIR open in battery feed to fuse
terminal.

"MA3" CHECK FOR VOLTAGE TO PCM Key off.

Install breakout box, leave PCM disconnected.

Measure voltage between PCM Test Pin 55 and ground.

Is B+ present?

Yes: RESTORE vehicle; CLEAR DTC, RERUN test. If code reappears, REPLACE the PCM.

No: REPAIR open in circuit 729 (F-Series) RESTORE vehicle.
CLEAR DTCs and RETEST.