97 Ford Expedition P0401 Code: Causes, Diagnosis, and Fixes for 5.4L V8 Engine

I recently connected an OBD2 scanner to my 1997 Ford Expedition Eddie Bauer Edition with a 5.4L V8 engine and received three identical diagnostic trouble codes: P0401. I've already replaced the fuel pump, fuel lines, fuel filter, crankshaft position sensor, and starter solenoid relay. Despite these repairs, the P0401 code persists. What other components or steps should I check to resolve this issue? Any guidance on possible causes of P0401 in a 97 Ford Expedition would be greatly appreciated.