Powertrain, generic (extended)

P279A Transfer Case Gear High Incorrect Ratio

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Range shift to high not completed by the shift motor
Worn shift fork or range sleeve slipping out of high
Implausible transfer case output speed signal
Mismatched tire sizes affecting the computed ratio

Symptoms

  • MIL and 4WD warning on
  • Unexpected low-range feel or loss of drive
  • Grinding from the transfer case

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–3,000
Time1–8 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

transfer caseplanetary gear setshift fork

Related codes

P279BP279CP2771P279DP2160

References: gsi.ext.gm.com, law.cornell.edu.