CAN network

U0103 — Lost Communication with Gear Shift Control Module

Severity: medium Drive with care, to a shop

What U0103 means

U0103 means one or more control units stopped receiving messages from the gear shift (shifter) module over the CAN bus. Normally this module continuously reports the selector position and receives commands, and the transmission and dash rely on that data. When the link drops, gear selection and the gear display may not work properly.

Symptoms

  • Check Engine or transmission warning light on
  • Unable to change gears or stuck in one gear
  • Gear position indicator flickers or is blank
  • Transmission in limp mode with limited gears
  • Shifter stuck in Park
  • Multiple warning lights at once
  • No start in severe cases

Possible causes

  1. Open, short, or high resistance in the CAN wiring between the control unit and shifter module
  2. Loss of power or ground at the gear shift module
  3. Corroded or loose connectors at the module
  4. Internal failure of the shifter module
  5. Low battery voltage or unstable charging
  6. Aftermarket remote start or alarm wiring interfering with the CAN bus
  7. Chafed or damaged wiring harness

How to diagnose

  1. Check battery voltage and charging output; charge or replace the battery if weak
  2. Run a full scan to see which modules are not communicating
  3. Check the fuses and relays that feed the shifter module
  4. Inspect the module connector and harness for corrosion, moisture, chafing, or tampering
  5. Test power and ground at the module connector with a multimeter
  6. Measure CAN-H and CAN-L voltages and resistance at the diagnostic connector (about 60 ohms between the lines is typical)
  7. If an aftermarket alarm or remote starter is installed, check its wiring and disconnect it per its instructions
  8. Unplug suspect modules one at a time only with a documented factory procedure

Common fixes

  • Repair or replace damaged CAN wiring and connectors
  • Clean and tighten ground and power connections
  • Remove or correct faulty aftermarket wiring
  • Update or reprogram the module software
  • Replace the gear shift module once it is confirmed faulty

Questions and answers

What does U0103 mean?

It means communication was lost with the gear shift module on the CAN bus. Other control units are not getting selector data from it, or cannot send it commands.

Can I drive with U0103?

Only to a shop, if the car moves at all. The transmission may stay in one gear or go into limp mode, and the car may fail to start.

Can a weak battery cause U0103?

Yes. Low voltage makes modules drop off the network and set U codes. Check the battery and charging system first, clear the code, and see if it returns.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.