Promload 03, from memory, has something to do with the voltages to the scanner, or a communication error revolving around something not being powered on.
Best bet would be to pull the CSC, DAQ, DSP, DCP, FEC, and FEB boards out of the system, then try to power it on. One of those boards could very well be faulty, dragging the power down to a necessary section that prevents the system from booting fully.
Another option would be to boot the system on, then check the voltage points at the side of the machine on the DBL (left side of machine if you're facing monitor, side panel must be off of the system). There will be some 5v, 7.5v, and 15v measure points... if any of those are dead or low, then chances are your Scanner Analog Power Supply has died.
Also, check the back of the system for any RED LED error codes (7 segment display, will show a number between 0-9 if any power errors have been reported to the PDB2).
Universal Diagnostic Solutions