P02D4: what cylinder 5 minimum offset learning actually measures

P02D4 concerns cylinder 5's learned injection-time adjustment, specifically a result below the minimum SQP authority limit. It should not be confused with a low injector control-circuit voltage.

The pressure measurement supports a time calculation

SQP makes a small injection during diesel fuel cut-off and estimates its quantity from fuel-rail pressure drop. On each map update, the controller compares the learned time delta with its minimum calibration. The published unit is microseconds; no universal numeric limit appears in this table.

This measurement chain gives a useful diagnostic distinction. The stored time correction is the conclusion, while the pressure-drop observation is evidence used to reach it. The code alone does not isolate which physical element caused the conclusion.

Completion depends on more than one good sample

The strategy stores saturation status for the SQP rail-pressure levels. A single saturated level can support P02D4, and all levels must be free of saturation for the described unset state. Review whether the affected region was actually relearned before calling a post-service result satisfactory.

The monitor also needs SQP authority diagnosis and injection management enabled; its timing follows the learning event rather than a fixed road-test duration. Type B, two-trip reporting is listed. The table supports interpretation of the monitor, while vehicle-specific professional procedures are still needed for any fuel-system test or repair.

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 1153.