Chassis, generic

C0399 4WD Mode Switch Circuit Malfunction

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C0399 cannot be traced publicly. Publicly available J2012 tables do not list it, PIP5094 from GM omits it, and the only matches in NHTSA's index are Mack recall numbers. Only the switch itself is therefore described. With the 4WD mode switch on the dash the driver chooses 2WD, Auto, 4HI or 4LO, and the transfer case module turns that choice into shift commands. GM does not file switch faults under C-codes: on the 2015 Colorado and Canyon its module stores B2725 when a switch with an Auto position is fitted to a truck without Auto 4WD and sends an invalid voltage, and B0770 for the Auto LED circuit; PIP5256 and PIP5256A order the correct switch. With a faulty switch signal the module usually ignores mode requests and keeps the current range.

Likely causes

Switch variant not matching the transfer case (e.g. Auto position without Auto 4WD)
Worn switch contacts or resistor ladder giving invalid voltages
Open or shorted lead between switch and transfer case module

Symptoms

  • Service 4WD message
  • Mode changes ignored or 4WD indicators flash

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–300
Time0.3–1.5 h

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

Parts involved

mode selector switchtransfer case control moduleswitch wiring

Related codes

C0388C0389C0387

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov.