U0100 – Lost Communication With ECM/PCM 'A'

What does U0100 mean?

A module has lost communication with the engine control module (ECM) or powertrain control module (PCM) over the vehicle's data network. Modern vehicles rely on constant module-to-module communication, so when the ECM stops responding, many systems lose the data they depend on.

What causes U0100?

Listed from most to least common:

1. 1Damaged CAN bus wiring — chafed, corroded, or broken
2. 2Loss of power or ground to the ECM/PCM
3. 3Corroded or loose connector at a module
4. 4Blown fuse feeding the ECM
5. 5Water intrusion into a connector or module
6. 6Failed ECM/PCM (less common than wiring)
7. 7Low battery voltage causing module dropout

Code breakdown

Common symptoms

• Check engine light on, often with several other warning lights
• Engine will not start, or stalls unexpectedly
• Multiple modules reporting communication faults
• Gauges and warning lights behaving erratically
• Scan tool cannot communicate with one or more modules

How to diagnose

1. 1Check battery voltage and condition first — low voltage causes communication dropouts that mimic module failure.
2. 2Check all fuses related to the ECM/PCM.
3. 3Verify power and ground at the ECM connector.
4. 4Inspect CAN bus wiring for damage, particularly where harnesses flex or pass near heat.
5. 5Look for water intrusion at connectors, a very common cause after flooding or a leaking seal.
6. 6Measure CAN bus resistance across the two bus wires — roughly 60 ohms is typical for a healthy high-speed network.
7. 7Check whether other modules also report U-codes, which helps locate the affected network segment.

This can cause a no-start condition or sudden stalling. Multiple systems may fail simultaneously. Treat as urgent.

Common fixes and estimated costs

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

Wiring and connection problems cause far more U0100 cases than actual module failure. Do not let a shop replace the ECM before the power, ground, and bus wiring have all been verified — module replacement is expensive and usually requires programming.

Related codes