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P0865 – TCM Communication Circuit Low (OBD2)

P0865 means "TCM Communication Circuit Low". The voltage the control unit measures in the transmission control unit power and communication circuit is below the permitted minimum. For a sensor this usually means the signal wire is shorted to ground or the reference voltage is missing; for an actuator, that its control wire is shorted to ground.

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P0865 quick summary

Fault codeP0865
MeaningTCM Communication Circuit Low
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityHigh
Can I drive?Carefully and only to a workshop – the gearbox may be in limp mode.
Repair cost€0 – €800

How the transmission control unit is powered and connected

These codes mean the transmission control unit's power supply, power relay or communication with other modules (gear shift module) is faulty.

The transmission control unit needs a stable supply through its power relay and communicates with the engine control unit and gear shift module over the CAN bus or dedicated lines.

Codes P0880–P0892 cover the TCM power input and relay; P0860–P0866 cover communication with the gear shift module and the TCM communication circuit.

Low battery voltage, corroded grounds, a faulty relay or wiring damage typically cause these codes, often together with limp mode.

On VAG vehicles (VW, Audi, Škoda, Seat, Cupra), VCDS and older VAG testers can show this code as the five-digit VAG code 17249 (P0865). Both numbers mean the same fault.

Symptoms of P0865

  • Gearbox in limp mode
  • No gear display or warning messages
  • Gear selector not responding
  • Communication codes in other modules
  • Check Engine or gearbox warning

Most common causes of P0865 (most likely first)

Mechanic's Pro Tip

Many gearbox limp-mode complaints with power or communication codes disappear after a new battery and cleaned ground points – check the basics before suspecting the gearbox.

  1. 1.Signal or control wire shorted to ground

    Most likely

    Look for chafed insulation where the harness touches the engine, exhaust brackets or body edges.

  2. 2.Missing 5 V reference or supply voltage

    An open reference wire or a shorted sensor elsewhere on the same reference circuit can pull it down.

  3. 3.Internal short in the transmission control unit power and communication

  4. 4.Weak battery or charging problem

  5. 5.Faulty TCM power relay or fuse

  6. 6.Corroded ground or supply connection

  7. 7.CAN bus or communication wiring fault

Step-by-step diagnostic guide

  1. 1

    Check battery and charging

    Measure the battery and charging voltage.

  2. 2

    Measure the signal voltage

    With the ignition on and the transmission control unit power and communication plugged in, back-probe the signal wire. Then unplug it and check the reference/supply and ground wires separately.

    Multimeter: Signal near 0 V = short to ground or no reference. Reference wire: about 5 V with ignition on. Signal wire to ground with connector unplugged: no continuity.

  3. 3

    Read all modules

    Read codes in all modules for communication and voltage faults.

  4. 4

    Check relay and fuses

    Check the TCM power relay and fuses.

  5. 5

    Measure supply and ground at the TCM

    Measure under load at the TCM connector.

    Multimeter: Supply near battery voltage, ground drop below about 0.1 V.

  6. 6

    Repair and verify

    Repair the supply, ground or communication fault and clear codes.

Frequently asked questions

Can I keep driving with P0865?

Carefully and only to a workshop – the gearbox may be in limp mode.

How much does it cost to fix P0865?

Typical repair costs are 0 € – 800 €, depending on the cause and the vehicle. The most common fixes are: signal or control wire shorted to ground; missing 5 V reference or supply voltage; internal short in the transmission control unit power and communication. Always confirm the cause with the tests above before buying parts.

Can a weak battery cause gearbox codes?

Yes. Low voltage can make the TCM reset or lose communication, causing limp mode.

Where is the TCM?

Depending on the car, in the engine bay, under a seat, in the footwell or on/inside the gearbox.

What is the difference between P0865 and similar codes?

P0865 belongs to a group of codes for the same component that differ in the type or location of the fault: P0862 – Gear Shift Module Communication Circuit High; P0863 – TCM Communication Circuit; P0864 – TCM Communication Circuit Range/Performance; P0866 – TCM Communication Circuit High; P0880 – TCM Power Input Signal; P0881 – TCM Power Input Signal Range/Performance. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.