P279A: transfer-case high-range command disagrees with measured ratio

P279A is a transfer-case command-state rationality check. The GM strategy asks whether the measured ratio looks like low range or neutral while the transfer-case controller commands 4WD high. The ratio is transmission output speed divided by transfer-case output speed.

A command-versus-result comparison

Monitoring requires transmission output speed above 500 rpm, engine torque above 80 Nm, engine speed above 300 rpm and accelerator position above 5%, with the listed brake condition. PTO operation and engine-power limitation inhibit the check, as do relevant speed and range-related faults. The source uses failure counts above 560 within sample counts above 800 at a 12.5 ms update rate. Reporting is Type A, one trip.

The numerical ratio windows in the table are overlapping and inconsistently grouped. This article therefore does not present them as a definitive standalone ratio test.

Diagnostic decisions

Preserve commanded transfer-case state and both measured shaft speeds. A selector display alone proves neither the attained range nor the validity of the calculated ratio. A bad speed input can undermine the comparison, which is why its fault exclusions matter.

Verification should establish a qualified high-range command and a credible attained-ratio result using the vehicle's service procedure. The code does not identify a failed actuator or justify forcing a range change while the vehicle is moving.

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 848.