P0866: Transmission Control Module Communication Circuit High
What to do
Expect a fixed gear. If the transmission is the only system reporting a communication fault, the problem is likely at its own connector — a considerably smaller job than a network fault.
What this code means
The transmission module's communication circuit is held high — a short to supply, or an open leaving the line at its idle voltage. An open silences only the module beyond the break, so a fault confined to the transmission points at its own connector rather than at the shared network. On a gearbox-mounted connector living in heat and road spray, corrosion is the leading cause.
Likely causes
Start here
- Open circuit in module wiring · moderate
- Corroded transmission connector · moderate
Less common
- Wiring shorted to voltage · moderate
- Faulty transmission control module · specialist
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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