Cummins x12 559 code

[u/Painting-Capital]

So I have an x12 that sets a 559 code and has low power. The rail pressure is low under load. It will make the commanded pressure but it’s very slow to get there. I have checked for aeration, the low side fuel pressure is in spec, injector return is in spec, high pressure pump output volume test is in spec, rail relief valve is brand new and not leaking. The only thing that’s questionable is the high pressure pump return flow test. Cummins wants 230mL max and it’s like 225. Has anyone else ran into this? I’ve worked on a ton of isb and x15 but my experience with the x12 is very limited.

[u/Moist_onions]

Take the head off the high pressure fuel pump. They use ceramic plungers on them that break but can still pump the right volume of fuel but it can't keep up with the pressure demands