ok first off no low fuel pressure code is acceptable the code was most likely 482
The fuel pressure is monitored by the ECM. If the rail fuel pressure is lower than an acceptable level for a given set of engine operating conditions, the fault code is logged.
NOTE: One possible cause of this fault code is a damaged or sticking drain-back valve located in the fuel supply circuit in the integrated fuel system module (IFSM). The IFSM is located directly above the fuel lift pump. You have an electronic lift pump not mech. This means you replaced the main fuel pump on the top of the isfm? and the electronic lift pump that runs for 2 min at key on is not changed? (verify operation, key on and listen ).You need to start from the beginning You should hook up a clear line on the supply side to the engine see if you have air there going into the engine. If needed bypass the tank lines ect by running the engine off a 5 gal bucket of fuel to duplicate the problem,
Integrated Fuel System Module (IFSM) mounting gasket leaking, These are also causes of excessive air in the return line. The head should be pressure tested