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U0100 code: lost communication with the engine control module (ECM)

Fault code U0100 means the other control modules in the car are not receiving messages from the engine control module over the CAN network. Because the gearbox, ABS and instrument cluster depend on it, several warning lights can come on at once, and the engine may stall or not start. The most common cause is a weak battery or a poor earth connection, not the module itself.

Check U0100

When the code is set

The control modules constantly send each other messages over the CAN network. When one of them (for example the gearbox or ABS module) does not receive the expected messages from the engine control module for some time, it stores U0100. The check engine light may come on and go off again, or not come on at all.

Can I keep driving?

Stop and do not drive on

Driving with this code is considered unsafe, because the engine can suddenly stall and not start again. Do not drive on; have the car taken to a garage.

Symptoms

  • The engine will not start, stalls while driving or goes into limp mode.
  • Several warning lights are on at once (ABS, stability control, airbags).
  • The radiator fans run at full speed.
  • A diagnostic tool cannot connect to the engine control module.
  • Higher fuel consumption.

Common causes

  • A weak battery or a charging fault.
  • A poor or corroded earth connection.
  • Moisture in connectors.
  • Aftermarket equipment (alarm, remote start, towbar wiring) interfering with the network.
  • A blown fuse or relay for the engine control module.
  • Damaged CAN network wiring.
  • A faulty engine control module (rare).

How to find the cause

A mechanic usually works like this:

  1. Test the battery at rest and under load, and check the charging system.
  2. Measure the voltage drop across the earth connections under load.
  3. Check the engine control module fuses and relays.
  4. Inspect connectors for moisture and corrosion, and remove any interfering aftermarket equipment.
  5. Scan all control modules to find which cannot be reached, and test the CAN network.

Other ways the code is written

Scan tools and workshop printouts sometimes show the code with extra characters:

  • U010000

Related codes

Frequently asked questions

Evoca fault code diagnostics is an informational aid and does not replace a professional inspection of your vehicle. Only an inspection of the vehicle at a workshop can confirm the cause of a fault.

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