My 2016 Peterbilt 579 with Cummins ISX throwing code 559 only when I go uphill with gas pedal on the floor (55mph-75mph). First both check engines light ( with the wrench and regular one) are on and if I put foot down from gas pedal check engine with wrench immediately disappear and only the other amber check engine stays on. If there are no hills for example 2 days regular check engine disappear also. I checked with computer and it is showing inactive code 559.
I tried to replace both filters , pressure relief valve and fuel pressure sensor . Nothing helps so far . Anyone knows what can be the issue? My fuel pump was rebuilt 70 000 miles ago . Now I got 580 000 miles on engine . Overhaul on engine was done at 535 000 miles .