P0800 OBD-II Trouble Code: Transfer Case Control System (MIL Request)

P0800 code definition

Transfer Case Control System (MIL Request)

What the P0800 code means

P0800 is an OBD-II generic code that the engine control module (ECM) got an input from the transfer case control module (TCCM) to turn on the malfunction indicator light (MIL).

This code is only used by the TCCM to activate the MIL through the ECM. The TCCM has to be accessed separately to diagnose additional codes.

What causes the P0800 code?

• The TCCM has a malfunction in one of the transfer case sensors or circuits and would set an internal separate fault code for the failure other than code P0800.

• The cause of code P0800 is only a pass through code to the ECM from the TCCM notifying it has a fault in the TCCM.

• Code P0800 is set by other manufacturer specific codes for transfer case problems. Not all manufacturers use code P0800 unless the TCCM is not on the vehicle's module network with the ECM.

What are the symptoms of the P0800 code?

• The Check Engine Light will be illuminated and the code is set in ECM memory as a TCCM fault request to turn on the MIL.

• The transfer case may have unknown issues since code P0800 is only an informational code and not the true fault code stored in the TCCM.

How does a mechanic diagnose the P0800 code?

• Scans codes and documents the code in the ECM.

• Scans for codes other than P0800 in the TCCM and diagnoses those codes as per the manufacturer's pinpoint test.

• Clears the ECM and TCCM code P0800 after repairs are done for any other TCCM code.

• Checks the catalyst and sensor for any signs of damage or leaking joints in the exhaust.

Common mistakes when diagnosing the P0800 code

• Clearing ECM memory codes before checking the TCCM for additional codes other than P0800
• Failing to clear ECM codes after the TCCM codes are fixed

How serious is the P0800 code?

• Since code P0800 only means that the TCCM has detected a fault in its system, the severity of the fault will depend on the codes that are existing on the TCCM.

• The Check Engine Light being on causes the vehicle to fail emissions testing.

• The TCCM may have serious problems that code P0800 only directs the mechanic to do further checks in the TCCM for other codes to diagnose.

What repairs can fix the P0800 code?

• Repairing any other codes received from the TCCM before clearing the P0800 code

Additional comments for consideration regarding the P0800 code

Code P0800 is only an informational code sent from the TCCM to the ECM to tell if there is a problem in the transfer case and to check the TCCM for detail codes it has set in the TCCM memory.

Need help with a P0800 code?

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