P0866: TCM Communication Circuit High

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.

Symptoms

• 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

Most likely causes

• Gearbox module without power or ground, or faulty
• Line between the modules open
• Line shorted to voltage
• Poor contact at a module connector

What to check, in order

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

1. Check the battery and grounds
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Clear the code and see if it returns
4. If it is a reference voltage code, disconnect sensors one at a time and see if the voltage recovers
5. Check whether a software update is available
6. 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 P0866 means

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

Frequently asked questions

Can I keep driving with code P0866?

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 P0866?

The most common causes are: gearbox module without power or ground, or faulty; line between the modules open; line shorted to voltage.

Related codes

• P0863TCM Communication Circuit
• P0864TCM Communication Circuit Range/Performance
• P0865TCM Communication Circuit Low
• U0101Lost Communication with TCM
• P0700Transmission Control System (MIL Request)

Sources

• saabwisonline.com: dtc p0866 tcm communication circuit high
• saabwisonline.com: dtc p0866 tcm communication circuit high
• law.cornell.edu: 13 CCR 1968.2

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.