@George kts
If you had mentioned in your question at first, that the board had been "reflowed" I wouldn't have posted the answer that I did.
Reflowing boards can cause problems if you get it wrong e.g. components losing connection with the board, components connections going short circuit because solder has flowed between them etc
You will need to get hold of the schematics for the motherboard (what is the make and model number of the board - not the HP part number - printed on the board) and then use them to trace the power rails to see what's happening.