CAN network

U0100 — Lost Communication with ECM/PCM

Severity: high Drive with care, to a shop

What U0100 means

U0100 means one or more control modules are not receiving messages from the engine control module (ECM/PCM "A") over the CAN bus. Normally modules constantly exchange data over a twisted pair of CAN High and CAN Low wires. The cause can be damaged wiring, missing power or ground at the ECM, low battery voltage, or a faulty module.

Symptoms

  • Check Engine light on
  • Engine stalls or won't start
  • Reduced power or erratic running
  • Harsh shifts or transmission limp mode
  • Several warning lights at once
  • Other U-codes stored together
  • Some module functions unavailable

Possible causes

  1. Damaged or open CAN bus wiring
  2. Loose or corroded module connectors
  3. Weak or dead battery, or charging problem
  4. Missing power or ground at the ECM/PCM
  5. Blown PCM fuse
  6. Another failed module pulling down the network
  7. Faulty ECM/PCM

How to diagnose

  1. Test battery voltage and the charging system
  2. Scan all modules and map which ones are not communicating
  3. Check ECM/PCM fuses and relays
  4. Verify power and ground at the ECM connector using the wiring diagram
  5. Inspect harnesses and connectors for chafing, moisture and corrosion
  6. Measure CAN bus resistance at the diagnostic connector (about 60 ohms on a healthy bus)
  7. Check CAN High and CAN Low voltages
  8. If needed, unplug suspect modules one at a time following a documented procedure

Common fixes

  • Repair CAN wiring and connectors
  • Replace or charge the battery and fix charging faults
  • Restore ECM power and ground, replace blown fuses
  • Replace a failed module that disrupts the bus
  • Replace and program the ECM/PCM with OEM software

U0100 on different brands

Ford · manuals
On Ford, U0100:00 may be set by a single module, such as the Explorer's power steering control module (PSCM), that lost communication with the ECM; the meaning depends on model and year.

Questions and answers

What does U0100 mean?

It means communication with the engine control module was lost on the CAN bus. Other modules get no data from it, so engine and transmission problems can follow.

Can I drive with U0100?

It's best not to, because the engine may stall at any time. If the car runs, drive carefully to a shop and keep trips short.

What should I check first?

Start with the battery, terminals, fuses and ECM grounds. Then inspect the CAN connectors and wiring and measure bus resistance.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.