A faulty 4WD mode switch that has worn contacts or internal corrosion, causing erratic or missing signals.; Damaged or corroded wiring and connectors between the switch and the 4WD control module, often from water intrusion or road debris.; An open circuit in the switch signal or ground circuit, such as a broken wire or a poor ground connection.; A short to ground or short to voltage on the switch signal circuit, which can be caused by chafed insulation.; A failed 4WD control module that is unable to correctly interpret the switch input.; A blown fuse or relay that supplies power to the 4WD switch or control module circuit.; Water or moisture intrusion into the switch housing or connector, leading to intermittent operation.

DTC codes / C03xx · Brakes, steering and suspension / C0399

C03994WD Mode Switch Circuit Malfunction

Q: What does code C0399 mean?

4WD Mode Switch Circuit Malfunction. C0399 indicates that the four-wheel-drive (4WD) control module has detected an abnormal condition in the electrical circuit for the 4WD mode switch, which is the driver-operated selector for choosing between two-wheel-drive and 4WD modes. The module monitors voltage or resistance on the switch signal circuit and sets this code when the reading falls outside the expected range for the selected position, indicating an open circuit, short circuit, or implausible signal.

C0399 indicates that the four-wheel-drive (4WD) control module has detected an abnormal condition in the electrical circuit for the 4WD mode switch, which is the driver-operated selector for choosing between two-wheel-drive and 4WD modes. The module monitors voltage or resistance on the switch signal circuit and sets this code when the reading falls outside the expected range for the selected position, indicating an open circuit, short circuit, or implausible signal.

SystemChassis

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaBrakes, steering and suspension

Symptoms you may notice

The 4WD or service four-wheel-drive warning light illuminates on the instrument cluster.
The 4WD mode selector switch may not respond when moved, leaving the vehicle stuck in its current drive mode.
The transfer case may fail to engage or disengage four-wheel drive, so the vehicle remains in two-wheel drive or stays locked in 4WD.
A message such as '4WD system fault' or 'check 4WD system' may appear on the driver information display.
Often no symptoms besides the warning light, especially if the fault is intermittent and the switch still functions correctly.
The 4WD indicator lamp may flash continuously or fail to illuminate when a mode is selected.

Common causes, most likely first

A faulty 4WD mode switch that has worn contacts or internal corrosion, causing erratic or missing signals.
Damaged or corroded wiring and connectors between the switch and the 4WD control module, often from water intrusion or road debris.
An open circuit in the switch signal or ground circuit, such as a broken wire or a poor ground connection.
A short to ground or short to voltage on the switch signal circuit, which can be caused by chafed insulation.
A failed 4WD control module that is unable to correctly interpret the switch input.
A blown fuse or relay that supplies power to the 4WD switch or control module circuit.
Water or moisture intrusion into the switch housing or connector, leading to intermittent operation.

How to diagnose and fix it

Verify the code with a scan tool and check for any related codes in the 4WD control module to confirm the fault is current.
Inspect the 4WD mode switch connector and wiring for corrosion, loose pins, or physical damage, and clean or repair as needed.
Test the switch with a multimeter to confirm it changes resistance or continuity correctly through each position according to the manufacturer's specification.
Check the switch ground circuit for good continuity to the chassis, and repair any high-resistance ground connections.
Measure voltage on the switch signal circuit at the control module connector while operating the switch to see if the module receives the correct signal.
If the switch and wiring test good, substitute a known-good switch or control module to isolate the faulty component before replacement.
Clear the code and perform a road test in each 4WD mode to confirm the repair and ensure the code does not return.

What the 4WD mode switch code hides

A common misdiagnosis is replacing the 4WD control module when the actual fault is a corroded connector at the switch, so always inspect the switch circuit first.
The switch may test fine with a multimeter but still set the code intermittently due to a broken wire inside the insulation that only opens when the harness flexes.
On some vehicles, the 4WD mode switch shares a ground with other dashboard components, so a bad ground elsewhere can trigger C0399 without any fault in the switch itself.
A weak or failing battery can cause the 4WD module to log this code if voltage drops during cranking, so check battery health before condemning the switch.
The code can be set by a mismatch between the switch signal and the actual transfer case position, which may indicate a transfer case motor or encoder fault rather than the switch.

Can I keep driving with C0399?

You can generally continue driving if the vehicle remains in two-wheel drive, but avoid relying on four-wheel drive until the fault is repaired. If the transfer case is stuck in four-wheel drive, avoid dry pavement and high-speed driving, as this can damage the drivetrain.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.