Technical Description of OBD-II Trouble Code
Malfunction Indicator Lamp (MIL) Request for Transfer Case Control System
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to many OBD-II vehicles (1996-newer), including Ford, GMC, Chevrolet, Dodge, Subaru, BMW, and others. However, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
If your vehicle has stored a code P0800, it means that the powertrain control module (PCM) has received a data input signal indicating a malfunction in the transfer case control system (TCCS), which requires the illumination of the malfunction indicator lamp (MIL). This code is only applicable to four-wheel or all-wheel drive vehicles.
The TCCS and transmission control module (TCM) are usually integrated into a single housing with the engine control module (ECM), although they may also be stand-alone units. This integrated unit 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 is connected to the tailshaft housing of the transmission and is responsible for transferring engine power to the rear differential and front differential. It also allows for power transfer to the front or rear driveshafts, or both.
The transfer case includes different gear ranges, such as 2HI, 4HI, 4LO, and NEUTRAL. The selection of the gear range can be done electronically via the TCCS/PCM or manually, depending on the vehicle. Vehicles with manually shifted transfer cases typically do not exhibit this type of code.
Similar to other automatic transmission/transfer case malfunctions, a transfer case control code can be caused by electrical or mechanical failure. If the PCM detects a TCCS malfunction, it will store a P0800 code and illuminate the malfunction indicator lamp (MIL).
A typical 4wd switch:
What is the severity of this DTC?
A stored code P0800 indicates the detection of a potentially serious TCCS malfunction. It is important to rectify the conditions that led to the storage of this code as quickly as possible.
What are some of the symptoms of the code?
Symptoms of a P0800 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P0800 troubleshooting steps?
When diagnosing TCCS codes, it is important to start by checking the cleanliness of the fluid and ensuring that the transfer case is filled to the correct level. If the fluid level is low, locate and repair the source of the leak. Refill the transfer case with the recommended fluid and proceed with the diagnosis.
To diagnose a code P0800, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and access to vehicle-specific diagnostic information.
Refer to your vehicle information source to find a technical service bulletin (TSB) that matches the year, make, and model of your vehicle, as well as the stored codes and exhibited symptoms. A TSB can provide helpful diagnostic information.
Once the transfer case is filled to the appropriate level with the recommended fluid, proceed with the following diagnosis steps:
Step 1
Use the scanner (connected to the vehicle diagnostic connector) to retrieve all stored codes and relevant freeze frame data. It is recommended to write down this information before clearing the codes and then test-drive the vehicle until the PCM either enters readiness mode or the code is reset.
If the PCM enters readiness mode at this time, the code is intermittent and may be more difficult to diagnose. In such cases, the conditions that led to the storage of the code may need to worsen before an accurate diagnosis can be made.
Step 2
If the code is immediately reset, the next step in the diagnosis requires searching your vehicle information source for diagnostic flowcharts, connector pin-out 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 output. Begin testing components and sensors related to the specific symptoms. Any components that fail to comply with recommended specifications should be considered defective.
Step 4
If no TCCS components have failed, use the DVOM to test the system circuits. The voltage drop testing method is effective for this task. Before testing the resistance with the DVOM, disconnect all controllers from the circuit.
