CM870 EGR Woes

[DadBunz]

I have a friend with a W900B that is an ISX truck. Original engine got pulled and a 03/06 CM870 was installed. He said that it hasn’t really had the power that it should since the swap and now it is only making max 20lbs boost and has EGR codes. 2791-7 was the active code and would come on every time the EGR would try its initial sweep in the first minute or two of running the engine. The light would go out as he would start driving until getting into a hard pull that would require boost. He complains of how lazy the throttle is like it is in a “safe” fuel map and maxes boost at 20psi. I pulled the EGR valve off per a Cummins tech thinking the valve is physically stuck. The valve is not physically stuck and is actually fairly clean. I put it back on and started checking wiring. 5v reference is steady, signal return has 0 resistance. I even checked the PWM with the multimeter and although not scientific, the meter cycled up and down so I believe the pwm is fine. Test drive and no change, light comes on and still only 20psi boost. Picked up a dorman EGR. Just as a test i plugged it in hanging off the side of the engine with the old egr still mounted to the exhaust. No codes, took it for a test drive still only 20psi boost. With the engine idling and warm the new egr valve hanging on the side sits there and cycles open and closed as i would imagine the ecm is commanding. Thought we had the check engine light figured out at least. Put the dorman egr on and barely made it out the shop door and check engine light came on. Took for test drive and was worse. Almost no boost, no power. I assume it went into derate this time. Checked the codes and it is showing 2791-5, 2791-7 and 27-5. So i hang the old EGR valve on the side and plug it in and it cycles once and throws the original 2791-7. I do not have Insite, only a basic Nexiq. The ECM is a used unit from a 2004 from the cummins shop. My question is, do i have stray voltage coming through the engine block causing these issues? Is it possibly a bad ECM? Bad new EGR that is shorted internally to ground? At this point I think the boost issue is unrelated because it struggles to make boost with the egr closed and no codes. Any insight*** would be appreciated or if anyone has ever chased ghosts like this before I need some pointers before we send it to sit at the cummins shop for weeks.. Trying to help a friend and it seems to be backfiring. Thanks