P0863 code meaning
TCM Communication Circuit
Powertrain system Generic code P08xx codes Check engine light
What P0863 means
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.
P0863 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P0863
- Check engine lamp on
- Gearbox warning or limp mode possible
- Gearbox faults not passed to the engine module
Common causes of P0863
- Most likelyWire or connector between engine and gearbox module open or corroded
- PossibleGearbox module without power or ground
- PossibleGearbox module faulty
- Less likelyInput of the engine module faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Transmission control moduleEngine control moduleSerial data lineModule wiring
What each part of P0863 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 8 | Area: transmission. |
| 63 | The exact fault in that area. |
What to do next
- Note P0863 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0863
What does P0863 mean?
P0863 means "TCM Communication Circuit". The TCM communication circuit is a direct link between gearbox and engine module outside the CAN bus. It is a generic code, so it means the same on every make.
Is it safe to drive with P0863?
Drive gently and only to a workshop. With P0863 the car can go into limp mode (reduced power) to protect itself.
Will P0863 fail an emissions test?
Yes, it can. P0863 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0863 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P0863 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.