P0863 – Transmission control module (TCM) communication


By Reinier (Contact Me)
Last Updated 2017-10-19
Automobile Repair Shop Owner

CodeFault LocationProbable Cause
P0863 Transmission control module (TCM) communication
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Wiring, poor connection, TCM -circuit malfunction

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What Does Code P0863 Mean?

OBD II fault code P0863 is a generic code that is defined as “Transmission control module (TCM) communication”, and is set when communication between the TCM (Transmission Control Module) and the PCM (Powertrain Control Module) or other control modules is lost.

Modern vehicles can have up to two dozen or more control modules, all of which are interconnected via the CAN (Controller Area Network) bus system. In simple terms, the CAN bus system consists of thousands of circuits and connections, and its function is to carry information to and from all control modules that are engaged in a particular function, such as the control/management of transmission shift patterns.

In practice, the TCM gathers information from a host of sensors and other control modules, such as the engine load/speed sensor(s), throttle position sensor, steering angle sensor and others, which information it communicates to the PCM via the CAN bus system. The PCM then uses this input data to calculate shift points before making suitable adjustment to the engine/fuel management systems in order to both modulate the engine power output to make gearshifts smoother, and to protect transmission components.

In terms of operation, all control modules have at least one other, closely related module to which it regularly reports its state of health via the CAN bus system. Thus, if a control module, in this case, the Transmission Control Module fails to report its state of health, the “companion” control module will set a relevant trouble code for the non-reporting control module.

Therefore, when input data from the TCM and/or other control modules is lost, the PCM is unable to control gearshift patterns effectively, and it will set code P0863 and illuminate a warning light as a result. In some cases, the PCM may initiate a failsafe or limp mode to limit engine power and to protect the transmission.

Where is the P0863 sensor located?

On most applications, the TCM is integrated into the PCM, and is therefore NOT user-serviceable. Note that this is true also for cases where the TCM is located outside of the PCM.

The image below shows a simplified schematic diagram detailing the basic interconnectedness of some of the major control modules in a modern application.

What are the common causes of code P0863 ?

Some common causes of code P0863 could include the following-

  • Note that unlike most other codes, control module failure is a common cause of this code
  • Damage to wiring, or open circuits that can be located almost anywhere in the CAN bus system

How expensive is it to fix code P0863 ?

Due to the many variables and large number of possible causes of this code, it is not possible to provide a repair estimate for this code that will be accurate for all, or even most applications.

What are the symptoms of code P0863 ?

Common symptoms of code P0863 could include the following-

  • Stored trouble code and an illuminated warning light
  • In some cases, and depending on the application, the shift pattern may be harsh, erratic, or unpredictable
  • In most cases however, it is more likely that the transmission will be locked into a failsafe or limp mode

What are common solutions to code P0863 ?

Common solutions to code P0863 could include the following-

  • Inspection and repair of damaged wiring
  • Replacement o defective control modules, but note that some control modules require programming and integration into the CAN bus system after replacement.
  • In rare cases, professional reprogramming of one or more control modules with manufacturer- supplied software fixes and/or updates

How serious is code P0863 ?

Code P0863 should be considered as serious, since the failure of one control module directly affects the operation of several other modules. If left unresolved, even if the vehicle is driveable, this could lead to the complete immobilization of the vehicle.

How safe is it to still drive the car with code P0863 ?

Since a vehicle on which code P086 is present can be immobilized unexpectedly, that vehicle should not be driven, and especially in traffic, since suddenly losing power on the vehicle can cause crashes.

How difficult is it to repair code P0863 ?

Non-professional mechanics should take note that this code is NOT repairable on a DIY basis, unless the cause of the code is obvious, such as visible damaged wiring or a disconnected ground strap/wire.

Also, take note that cheap, generic code readers can generally not access the CAN bus system; diagnosing CAN bus faults require dealer-grade diagnostic equipment, which includes an oscilloscope and a waveform library. Be aware that without advanced diagnostic equipment and suitable reference data, it may be impossible to diagnose this code, regardless of its cause.

When diagnosing this code, almost all non-professional mechanics are limited to a few basic diagnostic steps that may or may not resolve the code, which steps are outlined in the Troubleshooting section of this guide. If these steps do not resolve the issue, the better option is to refer the vehicle for professional diagnosis and repair, since some repairs typically involve testing hundreds, if not thousands of circuits individually. In some cases, the fault might never be found, in which case, all the wiring on the vehicle is replaced.

What are the common mistakes when repairing code P0863 ?

Common mistakes when dealing with this code typically include the following-

  • Disconnecting the battery to “clear” the code. Note that doing this has the potential to damage several other, unrelated control modules, or for the PCM to lose vital programming and/or memory, which can make it impossible to find the root cause of the code
  • Disconnecting electrical connectors and/or control modules in a random fashion in an attempt to isolate the fault. Note that doing this can also cause some control modules to lose programming or memory, which can also make it impossible to find the root cause of the code
  • Testing circuits in a random fashion in an attempt to stumble across the cause of the code. Note that not following prescribed testing methods can cause irreparable harm to the electrical system
  • Replacing control modules randomly in the hope that something “sticks”. While this might work in some cases, the fact is that doing this is almost always a waste of money and time

How do you troubleshoot code P0863 ?

Step 1

If a suitable code reader is available, record all fault codes present, as well as all available freeze frame data. This information can be of some use to identify the site(s) of intermittent faults later on.

Step 2

Check for the presence of system voltage related codes, since abnormal system voltages (both high and low) can damage control modules. If such codes are present, resolve the underlying cause before continuing with the diagnostic procedure.

Step 3

If the code appeared soon after routine servicing or other repairs, perform a thorough visual inspection of all wiring. Look for obviously damaged wiring, or wiring/connectors that appear to be shorted, disconnected, or corroded, since it is very easy to damage wiring during some types of repairs.

Include the battery terminals in this inspection- look for corrosion or terminals that are not properly fastened. Make repairs or replace wiring as required.

NOTE: Note that on many, if not most modern applications, it may be all but impossible to gain access to most of the wiring without removing engine components, carpets, trim panels, protective shields, and even the dashboard and seats. Thus, if all visible and accessible wiring is free of visible damage, but other codes are present, it is likely that the root cause of the fault is buried deep in inaccessible wiring. If this is suspected, the better option by far is to refer the vehicle to a specialist repair facility for professional diagnosis and repair. 

Step 4

The last step that is available to non-professional mechanics is to check that the PCM’s power supply is intact. Refer to the manual for details on the correct procedure to do this, but exercise extreme care during this step to avoid causing accidental short circuits that could destroy one or more control modules.

Note however, that issues with the PCM’s power supply will always be indicated by dedicated codes, which means that if such codes are not present, it is safe to assume that there are no issues with the PCM’s power supply.

Codes Related to P0863

  • P0864 – “TCM Communication Circuit Range/Performance”
  • P0865 – “TCM Communication Circuit Low”
  • P0866 – “TCM Communication Circuit High”


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