P0866: TCM Communication Circuit High OBD-II Trouble Code

In order to make your transmission function, a number of sensors, control modules, connectors and wires are used communicate data about the vehicle to the engine control unit. If an unusually high-voltage signal is detected on this circuit, the ECU will save a P0866 DTC.

Need a replacement transmission? Get an estimate for replacement transmissions and local installation. Look up your transmission model by vehicle make and model.

What Transmission Do I Have?

How Serious is the Code?

If the necessary sensors and control modules cannot communicate with the engine control unit, a number of serious drivability issues can occur. So a P0866 diagnostic trouble code is pretty serious.

Symptoms

Symptoms of a P0866 include:

  • Delayed or harsh shifts
  • Erratic shifting behavior
  • Limp mode

Causes

  • Damage to the wiring and connectors on the transmission wiring harness
  • Transmission control unit fault
  • Engine control unit failure
  • Software issue

How to Diagnose the Code?

When diagnosing a P0866 OBDII trouble code, the wiring and connectors on the transmission wiring harness will need to be inspected for signs of damage and corrosion. The manufacturer’s database of TSB’s should be checked for known fixes and software updates. If nothing has been found by this point, the TCM and ECU should be considered.

Need a replacement transmission? Get an estimate for replacement transmissions and local installation. Look up your transmission model by vehicle make and model.

What Transmission Do I Have?

What Repairs Will Fix P0866?

  • Repair or replace damaged wiring on the transmission wiring harness
  • Replace or repair broken/corroded connectors
  • Replace transmission control module
  • Replace engine control unit
  • Update TCM/ECU software

Related Codes

  • P0864 – TCM Communication Circuit Range/Performance
    P0865 – TCM Communication Circuit Low