Technical Description of OBD-II Trouble Code
Circuit Low in Four Wheel Drive (4WD) Switch
What does that signify?
This is a generic diagnostic trouble code (DTC) for the powertrain and is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from various manufacturers such as Chrysler, Ford, Jeep, General Motors, Dodge, Chevrolet, Mercedes, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration.
OBD-II trouble code P0838 and related codes P0836, P0837, and P0839 are associated with the circuit of the four wheel drive (4WD) switch. This circuit is also known as the transfer case control circuit.
The purpose of the 4WD switch circuit is to allow the driver to select the activation of the 4WD system and change the gear ratios of the transfer case as required, including two wheel high, two wheel low, neutral, four wheel high, and four wheel low, based on the configuration.
When the PCM or TCM detects voltage or resistance within the 4WD switch circuit that is too low, below the normal range threshold, code P0838 will be set and the check engine light, 4WD malfunction light, or both may be illuminated.
A four wheel drive selector switch:
What is the severity of this DTC?
The severity of this code is typically moderate, but problems with the 4WD switch circuit can be severe if the transfer case is stuck in low gear. This can cause unwanted stress on internal transmission and engine components, requiring immediate attention. In some cases, the vehicle may not move or shift into gear at all.
What are some of the symptoms of the code?
Symptoms of a P0838 trouble code may include:
What are some of the common causes of the code?
Causes for this P0838 code may include:
What are some P0838 troubleshooting steps?
The first step in the troubleshooting process for any malfunction is to research the Technical Service Bulletins (TSB’s) for the specific vehicle based on the year, model, and power plant. This can save a lot of time by providing guidance in the right direction.
The second step is to check the transmission and transfer case fluid to ensure it is at the appropriate level and in good condition. Then, inspect all components associated with the 4WD switch circuit for any obvious physical damage. Depending on the specific vehicle, this circuit may include several components such as the transfer case, switches, solenoids, the PCM, and TCM. Conduct a thorough visual inspection of the associated wiring for any visible defects like scraping, rubbing, bare wires, or burn spots. Next, check the connectors and connections for security, corrosion, and damaged pins. This inspection should cover all wiring connectors and connections to all components, including the PCM and TCM. Consult the specific technical data for the vehicle to verify the circuit configuration and determine if a fuse or fuse-able link is part of the circuit.
Advanced Steps
The advanced steps are highly specific to the vehicle and require advanced equipment for accurate performance. These procedures involve using a digital multi meter and referring to the specific technical references for the vehicle.
Voltage Checks
The reference voltage and acceptable ranges may vary depending on the specific vehicle and circuit configuration. The troubleshooting charts and appropriate sequence provided in the technical data will assist in an accurate diagnosis.
If this process reveals the absence of a power source or ground, continuity testing may be necessary to check the integrity of the wiring, connectors, and other components. Continuity tests should always be performed with the power removed from the circuit, and the normal readings for wiring and connections should be 0 ohms of resistance. Resistance or lack of continuity indicates faulty wiring that is open or shorted and requires repair or replacement. A continuity test from the PCM or TCM to the frame will confirm the serviceability level of ground straps and ground wires. The presence of resistance indicates a loose connection or possible corrosion.
What are some common repairs for this code?
Common Mistake
We always recommend seeking professional service to address this issue. However, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to be addressed. If you are interested in this option, please upload your ECU file to our portal.
