CAN network

U0403 — Invalid Data Received from Transmission Control Module (TCM)

Severity: medium Drive with care, to a shop

What U0403 means

A control module, often the engine computer, receives messages from the transmission control module (TCM) over the CAN bus, but the values are out of range or conflict with other data. These modules constantly share information such as vehicle speed, engine torque and gear selection. When the data is judged implausible, the module stores U0403 and may switch to a fallback strategy.

Symptoms

  • Harsh or erratic shifting
  • Transmission warning light on the dash
  • P0700 may be stored alongside it
  • Several warning lamps on at once
  • Functions of the affected module unavailable
  • Transmission stuck in one gear

Possible causes

  1. Failing TCM
  2. Internal transmission sensor fault corrupting the data
  3. CAN bus interference
  4. CAN wiring fault (open, short or chafed harness)
  5. Another faulty module loading the bus
  6. Connector or terminal corrosion
  7. Power supply or ground fault at a module

How to diagnose

  1. Run a full-module scan to see which modules are not communicating on the network
  2. Review freeze frame data and look for related codes such as P0700
  3. Check battery voltage and charging system output
  4. Measure CAN bus resistance and CAN-H/CAN-L voltages at the diagnostic connector
  5. Inspect the harness and connectors at the TCM and neighboring modules for chafing, moisture and corrosion
  6. Check power supply and grounds at the TCM and other modules
  7. Disconnect suspect modules one at a time, only following a documented procedure
  8. Test transmission sensors and check for software updates for the module

Common fixes

  • Repair the damaged CAN harness or corroded terminals
  • Fix power or ground faults at the module
  • Replace a faulty transmission sensor
  • Replace or reflash the TCM and program it to the vehicle
  • Replace another module that is overloading the bus

Questions and answers

What does U0403 mean?

Another control module received data from the TCM that it considers invalid. It points to a communication or data problem, not necessarily a failed transmission.

Can I drive with a U0403 code?

Only briefly and carefully, to get it diagnosed. The transmission may run in limp mode with harsh shifts, so don't put off the repair.

Is the TCM always the cause?

No. CAN wiring, corroded connectors, power and ground issues or a transmission sensor are common causes. Replace the TCM only after ruling these out.

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.