P0866 code meaning
TCM Communication Circuit High
Powertrain system Generic code P08xx codes Check engine light
What P0866 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. P0866 means the line stays high for more than 3 s with no start pulse, MIL after two trips. Over 10 V at the engine module pin points to a short to voltage; a voltage switching between 1 and 4 V is a healthy signal. A gearbox module without power sends nothing, so the pull-up keeps the line high and the gearbox runs in a fixed gear.
P0866 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 P0866
- Check engine lamp after two trips
- Gearbox in limp mode with fixed gear if its module has no power
- Gearbox faults not passed to the engine module
Common causes of P0866
- Most likelyGearbox module without power or ground, or faulty
- PossibleLine between the modules open
- PossibleLine shorted to voltage
- PossiblePoor contact at a module connector
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 P0866 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. |
| 66 | The exact fault in that area. |
What to do next
- Note P0866 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 P0866
What does P0866 mean?
P0866 means "TCM Communication Circuit High". 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 P0866?
Drive gently and only to a workshop. With P0866 the car can go into limp mode (reduced power) to protect itself.
Will P0866 fail an emissions test?
Yes, it can. P0866 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 P0866 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P0866 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.