What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code and applies to many OBD-II vehicles (1996-newer). This may include, but is not limited to, vehicles from Ford, GMC, Chevrolet, Dodge, Subaru, BMW, etc. Although generic, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
If your vehicle has stored a P0800 code, it means that the powertrain control module (PCM) has received a data input signal indicating that the transfer case control system (TCCS) has detected a malfunction requiring the malfunction indicator lamp (MIL) to illuminate. This type of code applies only to four-wheel drive or all-wheel drive vehicles.
The TCCS and transmission control module (TCM) may be standalone units but are most often integrated into a single housing with the engine control module (ECM). This is referred to as the PCM.
The PCM uses input signals from various engine, transmission, and transfer case sensors to calculate the automatic transmission shift strategy. The transfer case bolts onto the transmission output shaft housing. It is responsible for transferring engine power to the rear differential and front differential, respectively. The transmission output shaft provides power to the transfer case, which can be manipulated to transfer power to the front or rear driveshafts (or both).
The transfer case also contains a selection of gear ranges that typically include 2HI, 4HI, 4LO, and NEUTRAL. Just like power distribution, the transfer case gear range can be selected by pressing a button and controlled electronically (via the TCCS/PCM). Other vehicles must be shifted manually. Vehicles with manually shifted transfer cases typically do not set this type of code.
As with many other transfer case/automatic transmission malfunctions, a transfer case control code can be caused by an electrical or mechanical failure. If the PCM detects that a TCCS malfunction has occurred, a P0800 may be stored and a malfunction indicator lamp (MIL) will illuminate.
How severe is this DTC?
A stored P0800 code indicates that a potentially serious TCCS malfunction has been detected. The conditions that contributed to storing a code of this nature should be rectified as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P0800 fault code may include:
Inability to switch between 2WD and 4WD
Inability to switch between transfer case gear ranges
Flashing or constantly illuminated four-wheel drive or traction control indicator
Unusual noises from the transfer case or driveshaft
What are some common causes of the code?
Causes
of this code may include:
Faulty TCCS actuator or sensor
Internal (mechanical) failure of the transfer case
Shorts or opens in TCCS circuits
Faulty PCM or programming error
What are the troubleshooting steps for P0800?
When diagnosing TCCS codes, you should start by ensuring the fluid is clean and the transfer case is filled to the proper level. If the fluid level is low, locate the source of the leak and repair the condition. Refill the transfer case with the recommended fluid and proceed.
A diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle-specific diagnostic information will be needed to diagnose a P0800 code.
You can use your vehicle information source to locate a technical service bulletin (TSB) that matches the vehicle’s year, make, and model; as well as engine size, stored codes, and presented symptoms. If you find one, it may provide helpful diagnostic information.
With the transfer case filled to the proper level with the recommended fluid, move on to the first diagnostic step.
Step 1
Use the scanner (connected to the vehicle’s diagnostic connector) to retrieve all stored codes and relevant freeze frame data. It is a good idea to record this information before clearing the codes, then test drive the vehicle until the PCM enters readiness mode or the code resets.
If the PCM enters readiness mode at this point, the code is intermittent and may be much more difficult to diagnose. If this is the case, the conditions that contributed to storing the code may need to worsen before an accurate diagnosis can be made.
Step 2
If the code immediately resets, the next step in your diagnosis will require you to search your vehicle information source for diagnostic flowcharts, connector pinout charts, connector face views, and component testing procedures/specifications.
Step 3
Use the DVOM to test the voltage and ground circuits at the TCCS controller. If voltage and ground are detected, use the DVOM to check the controller’s output. Begin testing components and sensors related to specific symptoms. Any component not meeting recommended specifications should be considered faulty.
Step 4
If no TCCS components have failed, use the DVOM to test the system circuits. The voltage drop testing method works well for this task. Disconnect all controllers from the circuit before testing resistance with the DVOM.
TCCS-related codes are frequently attributed to the selector
In most cases, transfer case leaks are related to faulty seals
