P0800 – Transfer box control system, MIL request -malfunction

Benjamin Jerew

By Benjamin Jerew (Contact Me)
Last Updated 2016-01-10
ASE Master Tech

CodeFault LocationProbable Cause
P0800 Transfer box control system, MIL request -malfunction
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Wiring, mechanical fault

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What Does Code P0800 Mean?

It used to be that four-wheel drive was relegated to off-road vehicles, which meant that only military vehicles and sport utility vehicles could be found with a transfer case. The transfer case, of course, is the gear box that distributes transmission output between the front and rear axles. Interestingly, there are a lot of cars and trucks that are equipped with transfer cases, not for off-roading, but for performance and stability. After all, four wheels driving is better than two wheels driving. If your car says “4×4,” “4WD,” or “AWD,” it has a transfer case.

One of the major differences that has come about over the last couple decades is the switch from manually-operated transfer cases to electronically-operated transfer cases. While most 4×4 and 4WD vehicles required you to manually select a four-wheel drive mode, be it 4H or 4L (high and low ranges), via a second shift lever, many have switched over to fully automatic or push-button drive mode selection.

Of course, the switch to electronic controls, whether simply a center differential lock or fully-electronic range selection, means the addition of an electronic control unit, the transfer case control module (TCCM). The TCCM may or may not be part of the ECM or TCM (engine control module or transmission control module), depending on make and model. If the TCCM detects a malfunction, such as an out-of-range sensor or slow actuator, it may set DTC P0800, Transfer Case Control System Malfunction, in ECM memory, which prompts the ECM to illuminate the MIL or CEL (malfunction indicator lamp or check-engine light). Additional transfer case specific DTCs (diagnostic trouble codes) may also be stored in the ECM, TCM, or the TCCM itself.


What are the common causes of code P0800 ?

Depending on year, make, and model, DTC P0800 may have number of causes. Here are a couple of the most common.

  • Stuck Range Actuator – Some 4×4 vehicles use a push-button or dial-selector for choosing between 2WD and 4H and 4L. Electronically-controlled actuators take care of the rest. Some people simply don’t use the system, so the actuator never moves out of 2WD mode. Eventually, it rusts in place.
  • Vacuum Leaks – Some 4×4 systems used a mix of electronic and vacuum control systems. Some 4×4 vehicles with a vacuum-operated front-axle disconnect are notorious for vacuum leaks. Because they are exposed, the actuator may rust out or be damaged, and will no longer move the front axle disconnect fork.


What are the symptoms of code P0800 ?

Depending on the nature of the failure, you may or may not notice any drivability problems related to the MIL and transfer case. This is often the case for people who drive an electronic 4×4, but never use the system. Even so, the transfer case control module continually monitors sensors and actuators. At the very minimum, you will notice the MIL and reduced fuel economy, as the ECU is now running in “limp-home” mode.

For someone who regularly uses the 4×4 system, or whose fully-automatic system regularly kicks in for poor road conditions, such as rain or snow, the difference in performance may be immediately noticeable, again depending on the nature of the failure.


How do you troubleshoot code P0800 ?

It does one well to note that DTC P0800 is an informational code, only, and further diagnosis will be necessary to determine the TCCM-specific DTC and to make a proper repair. Other codes may be stored in ECM, TCM, or TCCM, but may or may not be accessible by some aftermarket scan tools. Make sure your scan tool software is completely up to date and 100% compatible with your specific year, make, model, and sub-model. Barring that, you may have to get access to a factory scan tool.

Codes Related to P0800

Because DTC P0800 is an informational code only, specific to the TCCM, expect to find at least one transfer case control system malfunction, which may include, but is not limited to:

  • P0802 Transfer Case Control System MIL Request Circuit/Open
  • P0818 Driveline Disconnect Switch Input Circuit
  • P0836 Four Wheel Drive (4WD) Switch Circuit
  • P0837 Four Wheel Drive (4WD) Switch Circuit Range/Performance
  • P0838 Four Wheel Drive (4WD) Switch Circuit Low
  • P0839 Four Wheel Drive (4WD) Switch Circuit High
  • P2771 Four Wheel Drive (4WD) Low Switch Circuit
  • P2772 Four Wheel Drive (4WD) Low Switch Circuit Range/Performance
  • P2773 Four Wheel Drive (4WD) Low Switch Circuit Low
  • P2774 Four Wheel Drive (4WD) Low Switch Circuit High