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Rawze

You say the engine only has 225k miles.. but if it has more than 8,000 hours of run time (on the fuel pump) on it then it is time to replace the guts in the fuel pump. Not knowing the truck, you need to pull the fuel pump apart and inspect it. Replace its guts while your in there. That way there is no question about it any more and it will be good for the next 8,000 hours (or 400k miles on a highway truck without PTO). It is the weak point in that model engine and if the fuel pump is going bad, then it will destroy the entire engine while only giving a low pressure code like you describe. ... It is nothing to play with.

Also, what needs to happen is that ypu need to do a rail leak-down test on it. See how much the fuel system is leaking internally. If it will not hold pressure in the rail properly, then it will over-work the pump, wearing it out faster, will cause efficiency losses, and if it is an injector, then it can damage a cylinder over time.
here is how to do an injector leak test...
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If it fails the leak-down test, then replace the pressure relief valve ... again ... and if it still fails ... just like 'Running Rough' mentioned above, pull all the fuel lines to the injecotrs and re-torque all the injector feed-tubes .. and then dothe test again. If it fails after that then it is either the pump or leaky injector(s). There is a procedure on quickserv.com on how to isolate it by capping off up to 3 injectors at a time and doing the test again. You need to follow that procedure where you cap off injectors to find the one(s) that are bad.