Fault code P0800 is called “Transfer Case Control System (MIL Request)” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.
Technical description and explained code P0800
This diagnostic trouble code (DTC) is a generic transmission code. Fault P0800 is considered a general code because it applies to all makes and models of vehicles. Although the specific repair steps may vary slightly depending on the model.
This fault indicates that the transmission control module (PCM) has received an incorrect data input. The data indicates that the transfer case control system (TCCS) has detected a fault that requires the Malfunction Indicator Light (MIL) to illuminate.
This type of code is only applicable to all-wheel drive vehicles equipped with a transfer case.
The PCM uses inputs from several engine, transmission, and transfer case sensors to calculate the automatic transmission shift strategy. The transfer case is responsible for transferring engine power to the rear and front differentials, respectively.
Like many other transfer case malfunctions, the code can be caused by an electrical or mechanical fault. If the PCM detects that a TCCS malfunction has occurred, code P0800 may be stored and the Malfunction Indicator Light (MIL) will illuminate.
Symptoms of vehicle malfunctions
The main signal that an error P0800 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.
It can also be warning signs such as:
- May have difficulty shifting between 2WD and 4WD.
- Unable to shift between gear ranges in transfer case.
- Blinking or constantly lit 4WD indicator light.
- Unusual noises from transfer case or driveshaft.
- Transmission may be put into emergency mode.
A stored code P0800 indicates that a potentially serious fault has been detected in the transfer case (TCCS). The conditions that contributed to the stored code should be corrected as soon as possible.
Factors that can cause this error code
The error code P0800 can mean that one or more of the following problems have occurred:
- Faulty transfer case actuator or sensor (TCCS).
- Short-circuited or broken wires.
- Internal (mechanical) malfunction of transfer case.
- Sometimes faulty PCM module is the cause.
How to fix or reset OBD-2 code P0800
Some suggested steps for troubleshooting and fix the error code P0800:
- Check wires for breakage, melting.
- Check the transfer case itself, as well as the sensor and actuator, for proper operation.
- Check all PCM / TCCS connectors one at a time.
- Test the PCM / TCCS modules.
After performing all of the above steps, remove fault code DTC P0800 with a scan tool. After that, you should test-drive the vehicle and check if the error will return.
Diagnose and repair of problems
The first thing to do when diagnosing a P0800 fault is to check the electrical part for damage. In today’s vehicles, electrical and electronic signals require good cabling for proper data transmission. Any faults, such as breaks, disconnected connectors, or corrosion, can interrupt signals, leading to transmission control failures.
Check the battery pack, as some PCM and TCCS modules are sensitive to low voltage. If the battery is low, the system may detect this as a failure. Make sure the battery is putting out at least 12 volts, and that the alternator is running properly, a minimum of 13 volts at idle.
Low ATF fluid levels can result in no pressure in the transfer case, which can also be a problem that TCCS recognizes as a failure. Fuses should not be overlooked either, as a blown fuse is a common problem.
On which vehicles does this problem occur most frequently
Fault code P0800 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:
- BMW
- Chevrolet (Suburban)
- Dodge
- Ford
- Mercedes (w211)
- Subaru
Fault code P0800 can sometimes be found with other errors. The most common are the following: P0700, P0802, P0898, P0899.
