i have a 2011 dodge 3500 megacab auto cummins with 180k has a H&S minni max tuner,5inch straight pipe with dpf delete,egr delete and intake.

yesterday the code p0191 popped up and now the truck won't rev past 1800rpm when in drive, but revs up fine in neutral. so it says its getting low fuel pressure. any idea what is causing this? i just had the fuel filter changed about a month ago

 

Sounds like the fuel rail pressure sensor to me has gone up. i would start there being it runs fine in neutral. If it was high pressure fuel pump i would think it would be acting up all the time.