P0628: fuel-pump relay control short-to-ground signature

The documented P0628 monitor evaluates the feed-fuel-pump relay high-side driver while it is powered on. A controller-specific abnormal voltage response indicates a short toward ground.

Low control voltage is not low fuel pressure

The threshold column characterizes a path of 0.5 ohm or less between the signal and controller ground. That is an electrical fault specification, not a pressure or pump-capacity limit. The code therefore points to relay-control integrity before it says anything about fuel delivery.

The monitor uses eight failed samples among ten, sampled every 250 ms, and lists Type A, one-trip reporting. The enable column names an 11 V run/crank condition but does not clearly state its comparison direction; vehicle-specific service information is required for that detail.

The on command is essential evidence

A low value while the output is not being requested can have a different meaning from the abnormal on-state response used here. Preserve the request state with the fault record rather than interpreting a standalone voltage measurement.

Verification should establish normal output behavior during an eligible command using the appropriate relay-circuit procedure. The table does not locate the short or prove that the fuel-pump motor itself has failed. It also does not authorize bypassing relay control, applying external power or measuring resistance on an energized circuit as a shortcut to diagnosis.

Source and applicability

2025 GM 25OBDG04B diagnostic parameter tables; the cited monitor only. Vehicle, engine and calibration applicability must be confirmed separately. These tables describe detection logic, not a complete repair procedure or a component replacement verdict. gm-2025-04b, PDF page 624.