P0863: TCM Communication Circuit

The TCM communication circuit is a direct link between gearbox and engine module outside the CAN bus. Subaru's 2005 WRX, sold as Saab 9-2X, uses a single 'AT diagnosis' wire: about once a second the gearbox module sends a frame with a start pulse of 40 ms low and 20 ms high, carrying among other things its request for the MIL. On CAN cars, loss of the gearbox module sets U0101 or U0402 instead. P0863 is the general fault of this link without telling range, low or high apart. No maker documents it on its own. Without the link the engine module receives no gearbox status, and a MIL request from the gearbox gets lost.

Symptoms

• Check engine lamp on
• Gearbox warning or limp mode possible
• Gearbox faults not passed to the engine module

Most likely causes

• Wire or connector between engine and gearbox module open or corroded
• Gearbox module without power or ground
• Gearbox module faulty
• Input of the engine module faulty

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the battery and grounds
2. Clear the code and see if it returns
3. If it is a reference voltage code, disconnect sensors one at a time and see if the voltage recovers
4. Check whether a software update is available
5. Do not replace the control module without a full diagnosis

Good to know

• Repair difficulty: medium
• It can trigger limp mode.
• It affects emissions.

What each character of P0863 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 8: Transmission
• 63: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0863?

It depends on the symptoms. If the car is running normally you can go to a repair shop; if it goes into limp mode, drive as little as possible.

What causes code P0863?

The most common causes are: wire or connector between engine and gearbox module open or corroded; gearbox module without power or ground; gearbox module faulty.

Related codes

• P0864TCM Communication Circuit Range/Performance
• P0865TCM Communication Circuit Low
• P0866TCM Communication Circuit High
• U0101Lost Communication with TCM
• U0402Invalid Data Received from TCM
• P0700Transmission Control System (MIL Request)

Sources

• law.cornell.edu: 13 CCR 1968.2
• saabwisonline.com: dtc p0865 tcm communication circuit low
• saabwisonline.com: dtc p0864 tcm communication circuit range performa

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.