Vehicle: 2022 GMC Sierra 2500, 4WD, V8-6.6L DSL Turbo, Automatic Transmission/Transaxle

Mileage: 66,871 Miles

Problem: The customer’s concern was the malfunction indicator light (MIL) was on. Symptoms included engine stalling, loss of power, and hard starting.

Case Details: The technician found diagnostic trouble code (DTC): P129F–Fuel Rail Pressure Too Low During Enable

He replaced the fuel pump driver module, but the DTC came right back along with the symptoms. It was a good time to call ALLDATA Tech-Assist.

The consultant advised that there were a few issues that could cause the problem:

1. A faulty fuel pump power control module (FPCM)--the most frequently cited cause for the P129F DTC, according to service bulletins and reports from owners.
2. Wiring and connector Issues. Inspect the wiring harness and connectors for the FPCM for any damage, corrosion, or loose terminals.
3. A failing in-tank fuel lift pump assembly. The problem could stem from an issue with the fuel lift pump itself. The test specifications were: At Idle: 4–8 PSI; Snap Throttle: should stay above 3 PSI; Under Load: 3 PSI
4. A clogged fuel filter. A restricted fuel filter can reduce fuel flow, leading to performance issues and potentially triggering related DTCs.

The technician followed the diagnostic advice and found that the in-tank fuel lift pump pressure was too low.

Confirmed Repair: The technician pulled the fuel tank, cleaned it, and replaced the lift pump. After clearing the DTC, he took the truck on a test drive. It ran like a champ. Problem solved!