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U0100 — Lost Communication Ecmpcm

Generic OBD-II · all makesNetwork / communication

Code U0100 indicates that one or more control modules have lost communication with the Engine Control Module (ECM) or Powertrain Control Module (PCM). This is a network communication error, typically occurring over the Controller Area Network (CAN bus).

Symptoms

What causes it — most likely first

1. Wiring Faults

Damaged, frayed, or shorted CAN bus wiring and corroded connectors preventing signal transmission.

2. Power Supply Issues

Blown fuses or failed relays providing power to the PCM, or a compromised ground connection.

3. Interfering Module

Another module on the network has failed internally and is 'pulling down' the entire communication bus.

4. PCM Internal Failure

Hardware failure within the Powertrain Control Module itself, rendering it unable to communicate.

How it's diagnosed

  1. Scan for all present DTCs to determine if communication is lost with only the PCM or multiple modules.
  2. Inspect and verify the integrity of PCM-related fuses and power relays.
  3. Check the PCM electrical connectors for corrosion, bent pins, or loose connections.
  4. Test for proper voltage and ground at the PCM harness.
  5. Perform a CAN bus voltage test to check for correct bias voltage on the High and Low lines.
  6. Isolate other modules on the network to see if communication is restored to the PCM.

Code facts & related codes

SystemNetwork / communication
StandardizationSAE generic — identical on every make
Related codesU0098 — Vehicle Communication Bus H LowU0099 — Vehicle Communication Bus H HighU0101 — Lost Communication TcmU0102 — Lost Communication Transfer Case ContrU0103 — Lost Communication Gear Shift Control U0104 — Lost Communication Cruise Control Modu

Frequently asked

Does a U0100 code mean I need to replace my PCM?

Not necessarily. While a failed PCM can cause this code, it is more common for the issue to be caused by wiring damage, blown fuses, or a different module interfering with the network.

Can a weak battery cause this communication error?

Yes, low system voltage during the vehicle's startup sequence can prevent modules from communicating correctly, which may trigger a U0100 code.

Is the vehicle safe to drive with this code?

Generally no. Because the PCM manages critical engine and transmission functions, a loss of communication can lead to sudden power loss or unsafe transmission behavior.