C0399 code meaning
4WD Mode Switch Circuit Malfunction
What C0399 means
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.
C0399 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of C0399
- Service 4WD message
- Mode changes ignored or 4WD indicators flash
Common causes of C0399
- 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
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Mode selector switchTransfer case control moduleSwitch wiring
What each part of C0399 means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 99 | The exact fault in that area. |
What to do next
- Note C0399 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about C0399
What does C0399 mean?
C0399 means "4WD Mode Switch Circuit Malfunction". C0399 cannot be traced publicly. It is a generic code, so it means the same on every make.
Is it safe to drive with C0399?
Drive gently and only to a workshop. With C0399 the car can go into limp mode (reduced power) to protect itself.
Will C0399 fail an emissions test?
C0399 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is C0399 to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will C0399 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.