P279ATransfer Case Gear High Incorrect Ratio
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- P279A fires when the transfer case should be in its high range, but the measured ratio says otherwise. GM explains the check in its 2019 ECM tables (section for the LCV engine): the ratio is calculated from transmission output speed divided by transfer case output speed, both from real sensors. If the control module commands 4WD high and the result falls in the window for low range or for neutral, the fault counts. It needs 7 seconds of failure within a 10-second window, output shaft speed above 500 rpm, positive engine torque and pedal input, no PTO operation and no active speed sensor faults. GM rates it Type A, so the MIL lights after one trip. Likely reasons are a range shift that did not complete, a worn shift fork or a slipping range sleeve. P279B and P279C cover the other two states.
- What you may notice
- MIL and 4WD warning on
- Unexpected low-range feel or loss of drive
- Grinding from the transfer case
- Likely causes
- LikelyRange shift to high not completed by the shift motor
- PossibleWorn shift fork or range sleeve slipping out of high
- PossibleImplausible transfer case output speed signal
- RareMismatched tire sizes affecting the computed ratio
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €150 – €3,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.