C03994WD Mode Switch Circuit Malfunction
- Generic OBD-II code · Chassis
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- Service 4WD message
- Mode changes ignored or 4WD indicators flash
- Likely causes
- PossibleSwitch variant not matching the transfer case (e.g. Auto position without Auto 4WD)
- PossibleWorn switch contacts or resistor ladder giving invalid voltages
- PossibleOpen or shorted lead between switch and transfer case module
- Repair
- Easy · Often possible to do yourself.Typical cost: €30 – €300
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.