EGR valve Failure

Been getting a egr delta pressure error when at low idle occassionally, idle up and it dissappears. then the engine derated, but the only code showing was a vehicle speed sensor. had a shop with computer take a look. he said they knew what the problem was. truck ended up back at our shop we replaced the DPF filter and sensors. we also replaced the turbo speed sensor and removed and cleaned egr valve no change.
so back to the computer. I found out the problem was acctually a EGR Valve failure detected code. We replaced the egr valve, no change.
It's a paccar mx13. When hooked back to the computer, the same code is still there. The guy thought since the valve is new maybe there's an issue in the wiring somewhere. So wondering what else we need to look at, or find a diagram for the wiring, connecters or whatever to help trace that stuff. Thoughts? also vehicle speed sensor error is still there, but speedo and everything works. Was told this is totally unrelated. ?