CAN network

U0157 — Lost Communication with Immobilizer Control Module

Severity: medium Drive with care, to a shop

What U0157 means

One of the control modules stopped receiving data from the immobilizer (anti-theft) module over the CAN bus. Normally the immobilizer reads the key's transponder chip and tells the engine computer that starting is allowed. If that communication is lost, permission is not given and the engine may not start.

Symptoms

  • Engine will not start, or starts and stalls right away
  • Security light flashes or stays on
  • Several warning lamps come on
  • Functions of the affected module are unavailable
  • Key is not recognized

Possible causes

  1. Failed immobilizer module
  2. Key not recognized (damaged chip, wrong key, weak reader antenna)
  3. CAN bus wiring fault: open, short or chafed harness
  4. Water damage to the security module or its connector
  5. Corroded connectors or terminals
  6. Faulty module loading the bus
  7. Power supply or ground fault at the module
  8. Low battery voltage

How to diagnose

  1. Check battery charge and terminals, since low voltage often causes false communication codes
  2. Run a full-module scan to see which modules do not respond
  3. Look for other U-codes to tell whether the problem is bus-wide
  4. Try a spare key and inspect the key reader antenna ring
  5. Check CAN bus resistance and voltages at the diagnostic connector
  6. Inspect the harness and connectors of the immobilizer module for corrosion, moisture or damage
  7. Check the module's power supply and grounds
  8. Disconnect suspect modules one at a time only using the manufacturer's documented procedure

Common fixes

  • Repair or replace damaged CAN wiring
  • Clean or replace corroded connectors and terminals
  • Restore the module's power or ground
  • Replace the reader antenna or key and perform key programming
  • Replace and program the immobilizer module
  • Replace a module that is loading the bus

Questions and answers

What does U0157 mean?

It means communication with the immobilizer module was lost. Other modules are not getting data from it over the CAN bus, so the anti-theft system may block engine starting.

Why won't my car start with U0157?

Without a reply from the immobilizer, the engine computer cannot confirm the key is valid and may block starting. Check the battery, try a spare key and inspect the wiring.

Can I fix U0157 myself?

Start with the battery, connectors and a spare key. If communication does not return, the CAN bus needs proper diagnosis, and replacing the module will require programming.

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.