P279B: transfer-case low-range command disagrees with measured ratio

P279B is the low-command counterpart to the transfer-case rationality test. The cited strategy compares a commanded 4WD-low state with a ratio derived from transmission output speed and transfer-case output speed. A ratio interpreted as high range or neutral is the fault concern.

Why both speed signals matter

The comparison depends on measured speeds, not just the requested range. Monitoring is qualified above 500 rpm transmission output speed, 80 Nm engine torque, 300 rpm engine speed and 5% accelerator position, with the listed brake condition. PTO and engine-power-limiting states are excluded. Several speed-sensor and related rationality faults inhibit reporting.

The counter uses more than 560 failed counts within more than 800 sample counts at 12.5 ms intervals. The row is Type A, one trip. Its detailed ratio-window presentation contains overlaps and ambiguous logical grouping, so a universal numeric acceptance band cannot be responsibly reconstructed here.

What to verify

Compare requested low range, actual speed-derived ratio and signal validity together. A completed shift request does not establish that the drivetrain attained the intended ratio, while an invalid speed signal does not prove a mechanical range failure.

Use the manufacturer procedure for controlled range testing and confirmation. The summary does not authorize unsafe moving-range changes, identify a particular broken component or establish safe continued operation from this code alone.

Source and application limits

This explanation covers the cited 2025 GM monitor. Confirm the vehicle and calibration in its service information before applying these limits; inclusion in this file does not establish model compatibility. GM diagnostic parameters, PDF page 849.