Powertrain, generic (extended)

P279B Transfer Case Gear Low Incorrect Ratio

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

P279B is the reverse case of P279A: 4WD low has been commanded, yet the transfer case behaves like high range or neutral. For a vehicle crawling off-road this is a real safety concern, because the expected reduction and engine braking are missing. In GM's 2019 ECM tables (LCV section) the module divides transmission output speed by transfer case output speed and flags the fault if the result lands in the high or neutral window. Timing and conditions match the high-range test: 7 s of failure within 10 s, output shaft above 500 rpm, engine torque and pedal applied, no PTO and no active speed sensor faults; Type A, so one failing trip turns on the MIL. A shift motor that stalls before the low position, worn planetary teeth or a broken fork are the likely culprits. P279C covers neutral.

Likely causes

Shift motor stopping short of the low position
Worn or damaged reduction planetary set
Broken or bent range shift fork
Faulty speed signal distorting the ratio

Symptoms

  • No reduction in 4WD low, weak crawling and engine braking
  • 4WD warning lamp and MIL
  • Clunking or grinding when selecting low range

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–3,500
Time1–10 h

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

Parts involved

transfer caseplanetary gear setshift fork

Related codes

P279AP279CP2771P2772P279D

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