P049B: insufficient low-pressure EGR in the air-flow model

P049B identifies insufficient low-pressure EGR through a model-based airflow comparison in this GM implementation. The controller compares estimated fresh-air mass with measured mass airflow and evaluates the resulting residual against an operating-condition threshold.

Restriction is the monitored interpretation

The source targets low-pressure EGR obstructions, but the calculation also depends on nominal cylinder flow and estimated high- and low-pressure EGR flows. A negative mean residual supports the insufficient-flow result; it is not a direct observation of a particular blocked passage.

The average covers 100 samples at 25 ms. Speed, fuel request and low-pressure EGR request must be sufficiently steady, and air control must be active with positive desired EGR rate.

A high low-pressure share is required

The continuation requires desired low-pressure EGR split above 0.90, valid cylinder and EGR flow estimates, and enable conditions sustained for at least 0.40 second. The combustion-mode support table excludes the displayed particulate-filter regeneration modes.

These details change verification: a drive with little low-pressure EGR demand or an excluded regeneration mode may never assess this fault. Preserve the requested split, airflow measurement and model-validity information with the test result. The diagnostic supports investigating a flow discrepancy, while the exact restriction or input error still requires vehicle-specific diagnosis. It does not prove that cleaning or replacing a valve will resolve P049B.

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 532; gm-2025-04b, PDF page 533; gm-2025-04b, PDF page 534; gm-2025-04b, PDF page 1702.