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P0839 Four Wheel Drive (4WD) Switch Circuit High

Check Engine Light

P0839 code definition

The P0839 code belongs exclusively to all wheel and four wheel drive vehicles. It indicates that the powertrain control module (PCM) has identified an issue within the four wheel drive switch circuit.

What the P0839 code means

An OBD-II generic code, P0839 is recorded when the transfer case control system reads an abnormal signal from the four wheel drive's actuator switch circuit. This is responsible for engaging or switching from the transfer case gear ratios in the two-wheel high setting, the two-wheel low setting, neutral setting, four wheel high, or four wheel low settings.

What causes the P0839 code?

A P0839 code may be due to:

  • Failure of the four wheel drive switch
  • Four wheel drive actuator failure
  • Defective senor
  • Malfunctioning connectors
  • Defective solenoid
  • Defective switches
  • Malfunctioning wiring harnesses

What are the symptoms of the P0839 code?

The P0839 code can trigger a range of symptoms that include no symptoms except for the check engine indicator to the total failure of the transfer case, which means the vehicle is inoperable. The other symptoms can include difficulty in shifting, failure to down or up shift, and engine stalling when the vehicle stops.

How does a mechanic diagnose the P0839 code?

The P0839 code is diagnosed with an OBD-II scanner and visual inspections. The mechanic first evaluates the freeze frame data to see when the code appeared, resets the fault codes and takes the vehicle for a road test. The purpose is to determine if the code reappears, and if so, to then move on to the visual inspection.

This inspection will identify any problems with the connectors or wires, and will also include repairing and replacing anything that is damaged, shorted, corroded or disconnected. The vehicle is then diagnosed again, and if the code reappears, the repairs are done.

Common mistakes when diagnosing the P0839 code

Because the P0839 code originates with the transfer case, one of the leading mistakes made during diagnosis is that the transfer case is the issue and must be rebuilt. This is a very expensive mistake, even if it is limited to component replacements rather than an entire rebuild.

How serious is the P0839 code?

A P0839 code is very serious because it means the vehicle may suddenly become inoperable, or incredibly difficult and unsafe to drive.

What repairs can fix the P0839 code?

The repairs needed to deal with the P0839 code will include:

  • Performing the initial diagnostic with the scanner, resetting the fault codes and checking to see if the code reappears after a road test. If it does, the technician does the visual inspection of the electrical systems, repairing and replacing anything needed, and retesting again.
  • Should the code reappear, it is important to find the vehicle manufacturer's wiring schematic (with views of the connectors). You will first check battery voltage on the input area of the four wheel drive switch. If there is no voltage, disconnect all control modules and look for connectivity between the fuse and input circuit on the sensor. You may need to repair short circuits.
  • If there is voltage, check the output side (the four wheel drive has to be on). If there is no voltage, you may need to replace the four wheel drive switch. Replace and retest.
  • If there is voltage on the input and output, test the voltage of the four wheel drive actuator. If you have no voltage, the circuitry continuity must be tested and the resistance between the PCM and the switch gauged. If could be that the PCM is the problem.

Additional comments for consideration regarding the P0839 code

Failure of the PCM is rare, but it is possible. If this is the issue, you will have to disconnect all of the modules, use the proper wiring diagram and replace all circuits needed. You will also have to remember that PCM replacement means reprogramming it to your vehicle's specifications.

Need help with a P0839 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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