U0100: Lost communication with engine control module (ECM/PCM A)

What U0100 means

The engine control unit (ECM or PCM) constantly sends data such as engine speed and temperature to other units over the data network. One of them, for example the gearbox unit or instrument cluster, has stopped receiving those messages for a while. It may be a one-off or a permanent fault.

Most common causes

• Weak battery or voltage dip while cranking
• Engine ECU supply fuse or relay
• Engine ECU connector loose, corroded or wet
• Damaged CAN network wiring between modules
• Poor engine ECU earth connection
• Failed engine ECU (less common)

Symptoms

• Several warning lights on at once
• Engine may not start or may cut out
• Rev counter or temperature gauge dropping to zero
• Automatic gearbox in limp mode

Can you drive with U0100?

With care If it appeared once after a weak-battery start and the car runs fine, you can drive carefully to a garage. If the engine cuts out, will not start or many warning lights come on, do not drive it.

How to check it

Check whether the code is active or only stored: if it is old and matches a day with a flat battery, clear it and see if it returns. Measure battery voltage with the engine off and while cranking.

How to read and clear U0100 with Final OBD

Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see U0100 with this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

• U0101 Lost communication with transmission control module (TCM)
• U0102 Lost communication with transfer case control module
• U0103 Lost communication with the gear shift module
• U0104 Lost communication with the cruise control module
• U0107 Lost communication with the throttle actuator control module
• U0121 Lost communication with the ABS control module
• U0126 Lost communication with the steering angle sensor module
• U0128 Lost communication with the park brake control module

U0100 is a generic code, so it means the same on any make, for example on Abarth, Citroën, Hyundai, Mini, Seat, Volvo. See what Final OBD does on your car in makes and models.