You have to think two fold. What is the ECM electrically commanding and what is the IPR doing. A few years ago I was towing my 5th wheel when all of a sudden my fuel pedal got real touchy and I heard the injectors chattering. A condition of when a tuner raises ICP. I hooked up my AE scanner and the ECM was commanding 700 psi which is the pressure at Idle. But the chattering told me otherwise. I then hookup 3000 psi mechanical gauge. A 5000 psi gauge would have been better but 3000 was all I had. As soon as I cranked the truck the 3000psi gauge maxed out. A stuck IPR was indicated. I replaced the IPR and ICP sensor. Was good to go.