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Error Code P0838 is defined as Four Wheel Drive (4WD) Switch Circuit Low. Error Code P0838 is a generic trouble code, meaning it applies to all vehicles with the OBD-II system, especially those made since 1996 up to present. This includes vehicles from, but not limited to, Chevrolet, Chrysler, Dodge, Ford, General Motors, Jeep, and Mercedes Benz. Specifications on the definition, troubleshooting, and repairs, of course, vary between make, model, and powertrain configuration.
This code is associated with the Four Wheel Drive (4WD) Switch Circuit, also known as transfer case control code. Other related codes include:
- Error Code P0836
- Error Code P0837
- Error Code P0839
The 4WD switch circuit allows drivers to choose the actuation of the vehicle’s 4WD system; it can change the transfer case gear ratio from two-wheel high, two-wheel low, neutral, four-wheel high, and four-wheel low, as required based on the configuration.
If the powertrain control module (PCM) or the transmission control module (TCM) detects a voltage or resistance that is too low below the normal expected value range within the 4WD switch circuit, then it will register the Error Code P0837 and activate the Check Engine light. The 4WD malfunction light may also be activated.
This code activates both the Check Engine light and the 4WD malfunction light
- Vehicle unable to get into gear
- Transfer case stuck in one gear
- Harsh transmission shift
- Transfer case malfunction
- Faulty 4WD switch
- Damaged or faulty wiring
- Defective or loose control module ground strap
- Corroded, faulty, or loose connector
- Defective fuse or fuse-able link (if applicable)
- Defective PCM or TCM
How to Check
As with most codes, you will need a source of vehicle-specific diagnostic information, diagnostic scanner, and a DVOM (digital volt/ohmmeter) to diagnose this error code accurately.
Start your diagnosis with a technical service bulletin (TSB), based on the specific year, make, model, engine size, symptoms, and power plant of the vehicle (if applicable). This can save a lot of time in the long run.
Next, check the level and condition of the fluid in the transmission and transfer case.
Then, find all the components associated with the 4WD switch circuit. Look for signs of physical damage. Depending on the vehicle, this circuit may incorporate several components, including switches, transfer case, solenoids, PCM, and TCM.
Check the associated wiring thoroughly; look for signs of burnt spots, bare wires, rubbing, and scraping.
Check the connectors and connections for security; make sure there are no corrosions, and damaged pins.
Check the wiring and connections of all the components, including the PCM and TCM. You can consult with the specific tech data of your vehicle to verify the configuration to see if a fuse or a fuse-able link is used into the circuit.
Advanced steps are usually vehicle specific and require advanced tools and equipment to perform, such as digital multimeter and the vehicle’s specific technical references.
The reference voltage and its acceptable ranges may differ from one vehicle and circuit configuration to another. Specific technical data also usually includes troubleshooting charts and the right sequence for an accurate diagnosis.
If the voltage check shows there is an absence of power or ground, then you need to check the integrity of the connectors, wires, and other components through a continuity test.
Make sure to remove the power from the circuit when conducting a continuity test. The normal reading for the wiring and connections is at 0 ohms of resistance. No continuity or resistance means there’s faulty wiring (open or shorted), which must be repaired or replaced.
To confirm the serviceability level of ground straps and ground wires, perform a continuity test to the PCM or TCM. Presence of resistance indicates loose connection or corrosion in the connector.
How to Fix
- Replacement of 4WD switch or solenoid
- Repair or replacement of faulty wiring
- Cleaning corrosion from connectors
- Repair or replacement of defective ground straps
- Replacement of blown fuse or fuse-able link (if applicable)
- Flashing or replacement of PCM or TCM
This code is considered moderate, but if the transfer case is stuck in low gear and cause unwanted stress on the internal transmission and engine components, then it’s considered severe and requires immediate attention.
In some cases, the vehicle may not shift or move into gear at all.
The common mistake in fixing this code is replacing the 2WD switch or the transfer case when the fault is in the wiring.