U0101 – Lost Communication With Transmission Control Module


There’s no communication between the transmission control module (TCM) and other control modules.

Main Symptoms:

One of the U0101 code's symptoms is the Check Engine Light
The Check Engine Light coming on might be an indicator of the U0101 trouble code
  • Check Engine Light comes on
  • Car doesn’t shift gears
  • Stays in one gear, usually 2nd or 3rd gear

Possible Causes:

  • Open in CAN bus + or – circuit
  • Short to power or ground in either + or – CAN bus circuit
  • Defective TCM (least likely)

Diagnostic Steps:

  • Using a bidirectional scan tool, control TCM and see if it responds. If it doesn’t probe it further
  • If it responds then check all the wires, connectors and fuses that make the circuit
  • With key on engine off, check the voltage of CAN C+ and C-. If the readings don’t match manufacturer’s specs then the communication circuits are bad

Read More: OBD2 Codes List: Meaning, Fixes, and Free Download