C0399
4WD Mode Switch Circuit Malfunction
The transfer case or 4WD control module has detected an invalid signal from the driver-operated 4WD mode selector. Mode requests from the dashboard may be ignored.
- Check engine light: Not always
- Emissions related: No
- Repair difficulty: easy
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
Common causes
Ordered by how often each turns out to be the cause.
- Mode selector switch failedCommon
- Wiring or connector faultPossible
Parts involved
- Mode Selector Switch
- Transfer Case Module
- Switch Wiring
C0399 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.