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ElectricalSeverity High: diagnose before driving on

U0100Lost Communication With ECM/PCM "A"

What it means

A control module stopped receiving messages from the engine control module (ECM/PCM) on the data bus (CAN). The engine control module (ECM/PCM) may be without power or ground, or the bus wiring to it is faulty.

Symptoms

  • Engine may not start; many warning lights
  • Several warning lights at once
  • Other modules may store related codes

Common causes

  • No power or ground at the engine control module (ECM/PCM)
  • CAN wiring open, shorted or corroded (CAN-H / CAN-L)
  • Low battery voltage during cranking
  • Failed engine control module (ECM/PCM)

What to check first

  1. Check fuses, supplies and grounds at the engine control module (ECM/PCM)
  2. Measure CAN-H to CAN-L resistance with the battery disconnected (about 60 Ω on most cars)
  3. Scope CAN-H and CAN-L for a clean mirrored signal
Can I drive with U0100? Not recommended. U0100 can lead to stalling, limp mode or engine damage; diagnose it before driving further.

Generic OBD-II meaning. Manufacturer-specific meanings and checks can differ; always check the service information for the car.

How to test the parts behind U0100

CAN bus networkWiring, typical values, multimeter and scope tests

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U0100: questions and answers

What does U0100 mean?

U0100: Lost Communication With ECM/PCM "A". A control module stopped receiving messages from the engine control module (ECM/PCM) on the data bus (CAN). The engine control module (ECM/PCM) may be without power or ground, or the bus wiring to it is faulty.

What are the symptoms of U0100?

Engine may not start; many warning lights. Several warning lights at once. Other modules may store related codes.

What causes U0100?

No power or ground at the engine control module (ECM/PCM). CAN wiring open, shorted or corroded (CAN-H / CAN-L). Low battery voltage during cranking. Failed engine control module (ECM/PCM).

How do you diagnose U0100?

1. Check fuses, supplies and grounds at the engine control module (ECM/PCM) 2. Measure CAN-H to CAN-L resistance with the battery disconnected (about 60 Ω on most cars) 3. Scope CAN-H and CAN-L for a clean mirrored signal

Can I drive with U0100?

Not recommended. U0100 can lead to stalling, limp mode or engine damage; diagnose it before driving further.