Error Code P0800 is defined as Transfer Case Control System (MIL Request). Error Code P0800 is a generic trouble code, meaning it applies to all vehicles with the OBD-II system, especially those made from 1996 up to present and equipped with automatic transmission. Specifications on the definition and steps for repairs may vary depending on the model and make of the vehicle.

This includes vehicles from, but not limited to, BMW, Chevrolet, Dodge, Ford, GMC, and Subaru. Specifications on the definition, troubleshooting, and repairs, of course, vary between make, model, and powertrain configuration.

If the Error Code P0800 appears, this means the PCM (powertrain control module) has received a data input signal that indicates that the TCCS (transfer case control system) has detected a malfunction, which requires the illumination of the MIL (malfunction indicator lamp). This type of code, however, is applicable to four-wheel drive or all-wheel drive vehicles only.

The TCM (transmission control module) and TCCS may be stand-alone units, but they are commonly integrated into a single housing with the ECM (engine control module), and this is what is usually referred to as PCM.

The PCM calculates the automatic transmission shift strategy by using input signals from multiple sources, such as the engine, transmission, and transfer case sensors. The transfer case bolts the tailshaft housing of the transmission. It transfers engine power to the front differential and rear differential, respectively. The transmission output shaft delivers power to the transfer case, and this can be manipulated to transfer power to either the front or rear driveshafts (or both).

The transfer case also has a selection of gear ranges, which typically include 2HI, 2HI, 4LO, and Neutral. Like the power distribution, the transfer case gear range may be chosen at a simple press of a button and then controlled electronically, either by the PCM or TCCS. For other vehicles, though, you need to shift manually. Vehicles with manually shifted transfer cases don’t usually have this type of error code.

As with other automatic transmission/transfer case malfunctions, an error code referring to the transfer case control may be caused by either electrical or mechanical failure. If the PCM determines a malfunction in the TCCS, then this error code will appear and illuminate the MIL.

Common Symptoms

  • Unable to shift between 2WD and 4WD
  • Unable to switch between transfer case gear ranges
  • Flashing or constant illumination of the 4WD indicator or traction control lamp
  • Unusual noises from driveshaft or transfer case

Possible Causes

  • Faulty TCCS actuator or sensor
  • Internal (mechanical) transfer case failure
  • Open or shorted TCCS circuits
  • Defective or programming error in PCM

How to Check

Before diagnosing this code, make sure the fluid is clean, and the transfer case is filled to the appropriate level. If there is not enough fluid (level too low), then there must be a leak. Find the source of the leak and do the necessary repair. Then, refill the transfer case with the require fluid before proceeding with the diagnosis.

You will need a diagnostic scanner, DVOM (digital volt/ohmmeter), and specific diagnostic information source to diagnose this code (i.e., Technical Service Bulletin or TSB) that matches your vehicle make, model, as well as engine size, code/s stored, and symptoms of the problem.

After filling the transfer case with the appropriate fluid at the recommended level, you can now start with the diagnosis.

First Step

Retrieve all stored codes and the pertinent freeze frame data by connecting the diagnostic scanner. You can write the information down before clearing the codes and take the vehicle for a test-drive until the PCM either enters readiness mode or the code is reset.

If the PCM sets to readiness mode, the code must be intermittent. This can be more difficult to diagnose, as the condition that caused the code may need to worsen before you can proceed with the diagnosis.

Second Step

If the code immediately resets, however, then you can get to the next step. Read your vehicle information source and familiarize yourself with the diagnostic flow-chart. Locate the connector pin-out charts, connector face views, and the specifications for the component testing procedures.

Third Step

Test the voltage and ground circuits at the TCCS controller by using the DVOM. If you can detect both voltage and ground, use the DVOM to check the controller output. Test the components and sensors related to the specific symptoms. Any components that fail to comply with the recommended specifications are probably defective.

Fourth Step

If the TCCS components are in good condition, then test the system circuits using the DVOM. You can use the voltage drop testing method for this step. Make sure you disconnect all controllers before testing for the resistance with your DVOM.

How to Fix

Codes received by the TCCM must be cleared before clearing this code.

  • Replacement of defective components such as TCCS actuator or sensor
  • Repair or replacement of open or shorted TCCS circuits
  • Replacement and reprogramming of PCM (rare

If this code is stored, this could mean there’s a potentially serious problem in the TCCS. Conditions attributed to a code of this nature must be addressed as soon as possible.

TCCS related codes are usually attributed to the selector switch. In most cases, transfer case leaks are related to defective seals.