The P1870 series codes address four-wheel drive systems and transfer case control found in trucks, crossovers, and SUVs from Asian manufacturers including Toyota, Nissan, Honda, Mazda, and others. These codes relate to the electrical and electronic control aspects of 4WD systems.
Transfer Case Control Systems
Modern 4WD systems use electronic control modules to manage transfer case operation, engagement of front axle hubs, and torque distribution between axles. The Electronic Control Module (ECM) or a dedicated 4WD control module processes input from mode switches, speed sensors, and wheel slip sensors to determine when and how to engage 4WD.
Part-time 4WD systems use a transfer case with gear sets that mechanically couple front and rear driveshafts when engaged. On-command 4WD systems use clutches and solenoids to progressively engage 4WD, allowing some slip between axles for smoother operation. Full-time 4WD systems always distribute torque to all wheels but vary the ratio based on conditions.
Electronic Shifting Mechanisms
P1871 and P1875 relate to transfer case shift motors and encoder motors that physically move the transfer case between 2WD, 4WD High, and 4WD Low ranges. These are typically gear reduction motors with built-in position sensors or encoders that tell the control module the transfer case current position.
When shifting problems occur, the diagnostic approach differs based on whether the issue is mechanical (binding in the transfer case) or electrical (motor, circuit, or control module). Intermittent codes suggest electrical issues, while consistent mechanical symptoms may require transfer case disassembly.
Hub Lock and Clutch Solenoids
P1872 and P1873 address electrically-actuated front axle hub locks and clutch solenoids that engage the front wheels. Automatic 4WD systems engage hubs and clutches based on wheel slip detection, without driver input. These systems use solenoid valves that route hydraulic pressure to lock the hubs or apply clutch packs.
When diagnosing hub lock codes, verify battery voltage at the solenoid connectors and check solenoid resistance to ground. The hubs themselves can fail mechanically due to contamination, corrosion, or wear. Automatic locking hubs may fail to disengage, causing driveline binding and increased fuel consumption.
4WD Performance Switch Diagnostics
The P1874 code relates to the 4WD performance switch or mode selector, which allows the driver to choose between 2WD, Auto, 4WD High, and 4WD Low modes. These switches are prone to contamination and wear, especially in vehicles used off-road or in harsh environments.
Using a multimeter, test switch resistance in each position against manufacturer specifications. The switch grounds circuit through multiple positions, and resistance values indicate which range the switch is currently selecting. Some switches use hall-effect sensors rather than contact-style variable resistance.
System Integration
4WD systems integrate with vehicle stability control, anti-lock braking, and traction control systems. When wheel slip is detected, the stability control module may command transfer case clutch engagement via CAN bus communication. This integration means codes in other systems can cause 4WD fault codes to set.
On 2015 vehicles, the 4WD control module may share inputs with the ABS module. Speed sensor failures, communication faults, or module issues in the ABS system can cascade into 4WD fault codes. Proper diagnosis requires understanding this system interaction.
Transfer Case Service
Transfer case electrical components are typically accessible without removing the transfer case from the vehicle. Shift motors, position sensors, and solenoids can be tested and replaced individually in most applications. However, mechanical transfer case issues (worn gears, bearing failure, chain stretch) require transfer case removal and teardown.