Reference meaning
What does this code mean?
A reporting module did not receive expected messages from the engine/powertrain controller. Record which modules report U0100 and which still communicate. Battery voltage, power supplies, grounds, connectors and network wiring should be tested before condemning a controller. The reporting module is not necessarily the failed module.
Sources and interpretation [1]
A code identifies a detected condition, not a part that must automatically be replaced.
Narrow by vehicle
A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
Before replacing a part
Record codes and freeze-frame data. Check battery condition, related codes and visible wiring or hose damage. Follow the vehicle's diagnostic procedure and test results before replacing parts.
Scan worksheet →Manufacturer wording
Source wording can vary by vehicle and suffix. It is shown as written, not as a universal diagnosis.
U0100Lost Communication With ECM/PCM
U0100Lost Communication With ECM/PCM
U0100-00Lost Communication With ECM/PCM
Manufacturer communications
128Original wording in English
MAZDA
DESCRIPTION Some CX-70 and CX-90 PHEV vehicles may fail to start (enter READY mode) in cold weather, but the vehicle can often be started successfully on the next attempt. This may occur when leakage current between connector terminals causes the MAIN Relay to be falsely detected as ON. As a result, the Battery ECU may incorrect…
HYUNDAI
Certain Palisade Hybrid (LX3 HEV) vehicles may illuminate the Malfunction Indicator Lamp (MIL) and display a false “Check Hybrid System” warning message after use of Bluelink Remote Start following extended parking periods. Related DTCs associated with the MIL include U116087, U016487, U100787, U012987, U010087, U100387, U100287…
HYUNDAI
Certain Palisade Hybrid (LX3 HEV) vehicles may illuminate the Malfunction Indicator Lamp (MIL) and display a false “Check Hybrid System” warning message after use of Bluelink Remote Start following extended parking periods. Related DTCs associated with the MIL include U116087, U016487, U100787, U012987, U010087, U100387, U100287…
JEEP
Trailer Tow Module (TTM) Setting Active DTC U0100-00 – Lost Communication With ECM/PCM
Check vehicle applicability
564A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
| Make | Model | Model year |
|---|---|---|
| Audi | A4 ↗ | 2019 |
| Audi | A4 ↗ | 2010 |
| Audi | A4 ↗ | 2009 |
| Audi | A4 ALLROAD ↗ | 2014 |
| Audi | A4 ALLROAD ↗ | 2013 |
| Audi | A4 AVANT ↗ | 2012 |
| Audi | A4 AVANT ↗ | 2011 |
| Audi | A5 ↗ | 2019 |
Sources and interpretation
These explanations are editorial summaries of the linked manufacturer material. PDF page numbers count from the first file page. Original publications are in English; translations are reference aids. A cited edition is not a claim that it is the manufacturer's latest instruction.
- Audi TT Quick Reference Specification Book · October 2013 ↗
Audi of America · PDF page(s) 143 · 2013-10
- 01-19-05TT / 2054867/7 · No start or ECM communication, U0100 ↗
Volkswagen Group of America · PDF page(s) 1, 2 · 2024-07-11
Source check date · 2026-10-27
Can I keep driving?
A code alone cannot establish whether driving is safe. A flashing malfunction light, severe shaking, overheating, oil-pressure warning or loss of braking or steering needs prompt attention. Stop in a safe place and seek assistance when control or engine safety is affected. Follow the vehicle handbook.