low fuel rail press 559 code

[Rawze]

You inspected the pump,... good move to do that for the prevention of the dreaded pump failure of engine death.

The high pressure relief valve at the end of the fuel rail are notorious for being a second cause for what you describe. I would have replaced it to see if the code goes away before jumping to higher conclusions with the pump.

Also there is the pressure control regulator, partially blocked pick-up lines, or other things too.

Maybe also watch "commanded" pressure vs "measured" pressure and see how much differences there are.