P0864 code meaning
TCM Communication Circuit Range/Performance
Powertrain system Generic code P08xx codes Check engine light
What P0864 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. P0864 means signals arrive but in the wrong format: Subaru sets it after 60 s without a valid frame while driving above 500 rpm and 10.9 V, MIL after two trips. The service procedure checks shifting, then interference from aftermarket radios with bad grounds, then replaces the gearbox module. GM's 2005 2.8 and 3.6 V6 report it with U0101 when gearbox messages are missing for 2.5 s.
P0864 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 P0864
- Check engine lamp after two trips
- Shift problems or limp mode possible
- Often no noticeable effect
Common causes of P0864
- Most likelyGearbox module faulty or its software
- PossibleInterference from retrofitted radio or phone with poor ground
- PossibleLoose contact in wire or connector between the modules
- Less likelyLow supply voltage
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 P0864 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. |
| 64 | The exact fault in that area. |
What to do next
- Note P0864 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 P0864
What does P0864 mean?
P0864 means "TCM Communication Circuit Range/Performance". 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 P0864?
Drive gently and only to a workshop. With P0864 the car can go into limp mode (reduced power) to protect itself.
Will P0864 fail an emissions test?
Yes, it can. P0864 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 P0864 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P0864 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.