Fault codeP0865

MODERATE

TCM Communication Circuit Low

What this code means

TCM Communication Circuit Low.

P0865 is a generic code: it means the same thing on every make. P0865 belongs to family P08xx — Transmission (continued). Its page explains how to read a code and groups all its codes by part.

Type of fault

The control unit reads a voltage on this circuit below its expected range. Usual causes: a wire shorted to ground or broken, a corroded or loose connector, or the component itself. Check the wiring and connector before replacing the part — that is where many of these faults turn out to be.

Gearbox control unit (TCM)

The gearbox control unit drives the solenoids and reads the gearbox sensors. It communicates with the engine control unit over the vehicle network and is fed by its own relay and fuse.

What you notice

The gearbox goes into limp mode, sometimes several warnings at once, shifts that do not happen.

How to check it

Check its fuses and relay, then the battery voltage and the grounds: a weak supply causes many of these faults. Then the connectors of the unit, which on some cars sits on the gearbox itself and suffers from heat and leaks.

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
0standardised code
8Transmission (continued)
65P0865