P0865 code meaning
TCM Communication Circuit Low
Powertrain system Generic code P08xx codes Check engine light
What P0865 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. P0865 means the line stays low for more than 3 s, MIL after two trips. The engine module pulls the line up; the procedure measures below 1 V at the engine module pin with ignition on, checks the harness for a short to ground and then reads the gearbox codes or replaces the gearbox module.
P0865 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P0865
- Check engine lamp after two trips
- Gearbox faults not passed to the engine module
- Shift problems possible
Common causes of P0865
- Most likelyLine between the modules shorted to ground
- PossibleGearbox module faulty, holding the line low
- PossiblePoor contact at engine or gearbox module connector
- Less likelyPull-up in 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 P0865 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. |
| 65 | The exact fault in that area. |
What to do next
- Note P0865 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 P0865
What does P0865 mean?
P0865 means "TCM Communication Circuit Low". 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 P0865?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0865 fail an emissions test?
Yes, it can. P0865 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 P0865 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P0865 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.