4WD Low Switch Circ


The PCM has detected a problem in the four wheel drive low switch circuit. A code has been stored and the service engine light illuminated. Only all-wheel drive or four-wheel drive vehicles should exhibit this code. If this code is presented in a two-wheel drive vehicle, suspect a defective PCM or a programming issue.

Code Set Parameters

In order for this code to be exhibited, the transfer case control system must detect an abnormal reading from the four wheel drive actuator switch circuit. The four wheel drive switch is responsible for actuation of the four wheel drive system and changing between transfer case gear ratios (two-wheel high, two-wheel low, neutral, four-wheel high, and four-wheel low).


Symptoms can range from none at all to total transfer case failure (the kind that results in a stranded motorist condition). Some of the most common symptoms may include service engine soon lamp illumination, harsh shifting, failure to shift (from two to four-wheel drive low or high), or engine stall when coming to a stop.

Common Causes

Common causes include four-wheel drive switch or actuator failures. Other causes include faulty sensors, solenoids, or switches, and defective wiring harnesses and connectors.

Common Misdiagnosis

Technicians report that many transfer cases are totally rebuilt (at great cost to the vehicle owner) or replaced when component replacement may repair the problem. Many repair facilities opt to totally rebuild or replace transfer cases instead of repairing them. This is a viable option, particularly in high-mileage vehicles, and also helps them to cover themselves from additional failures during the perceived warranty period.


  • A scanner (or code reader) and a digital volt/ohmmeter will be helpful when diagnosing this code. This type of code applies to all-wheel drive or four-wheel drive vehicles only
  • If this code is present in your two-wheel drive vehicle, suspect a faulty PCM or programming error. In most four wheel drive applications the four-wheel drive switch is used to switch between two and four wheel drive operation, as well as between low and high ranges
  • Some applications use fully electronic systems of actuating these changes and others utilize a combination of engine vacuum and electronically controlled solenoids to effect the changes
  • Other systems use electronically controlled solenoids to lock the front hubs into place and a system of manual levers to control shifting of the transfer case
  • Still other (all wheel drive systems) change from two wheel drive to four wheel drive automatically depending upon road conditions
  • Virtually all OBD II equipped vehicles utilize some type of electronic transfer case control module to affect transfer case and front differential changes as well as to interact with the other control modules. While the four-wheel drive switch may vary by design from one manufacturer to another, it is usually of the “on/off” contact variety and is stationary mounted near instrument panel, within the driver’s reach
  • Begin your diagnosis with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • After the codes are cleared, operate the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • If the code immediately returns, obtain a manufacturer’s wiring schematic with connector views and check for battery voltage on the input side of the four wheel drive switch using the digital volt/ohmmeter
  • If there is no voltage present on the input side of the switch, disconnect any related control modules and check for continuity between the system fuse and the input circuit at the clutch position sensor
  • Repair open or shorted circuits as required
  • If voltage is present on the input side of the four wheel drive switch, use the scanner to command the four-wheel drive system “ON” and check for voltage on the output side of the switch
  • If you have voltage on the input side and none on the output side (when four-wheel drive is commanded) suspect a faulty four-wheel drive switch
  • Replace the switch as needed and retest the system afterward to ensure a successful repair. If there is voltage present on the output circuit of the clutch position sensor, check for voltage at the four-wheel drive actuator
  • If no voltage is present, test circuit continuity and resistance between the switch and the PCM and compare your findings with manufacturer’s specifications
  • Take care to disconnect all related control modules before testing
  • Again, refer to the vehicle wiring diagram to ensure that all circuits are tested
  • Repair or replace system open or shorted circuits/connectors as required
  • If all circuits coincide with manufacturer’s specifications, suspect a defective PCM
  • Remember that PCM failure is rare and replacement will require reprogramming.