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.
Manufacturer wording
Source wording can vary by vehicle and suffix. It is shown as written, not as a universal diagnosis.
U0100Lost Communication With ECM/PCMU0100Lost Communication With ECM/PCMU0100-00Lost Communication With ECM/PCMU0100Lost Communication With ECM/PCMView all · Manufacturer wording →
Manufacturer communications
128Original wording in English
MAZDA
Reported code forms U0100
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…
HYUNDAI
Reported code forms U010087
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…
HYUNDAI
Reported code forms U010087
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…
JEEP
Reported code forms U0100-00
Trailer Tow Module (TTM) Setting Active DTC U0100-00 – Lost Communication With ECM/PCM
FORD
Reported code forms U0100
Some 2021-2022 Mustang Mach-E vehicles may exhibit a "Parking Brake Failure, Service Immediately message displayed in the instrument panel cluster (IPC) and diagnostic trouble codes (DTCs) C2007, C2008, C1034, U0293…
FORD
Reported code forms U0100
Some 2023-2025 F-Super Duty vehicles may exhibit a no crank/no start or intermittent no crank/no start with multiple warning indicators and DTC U0100 stored in the anti-lock brake system (ABS), audio front control…
Check vehicle applicability
565A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
Historical vehicle association; OBD-II compatibility is not established by this record.
| 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 |
| Audi | A5 | 2014 |
| Audi | A5 | 2013 |
| Audi | A5 | 2012 |
| Audi | A5 | 2011 |
Historical records · 1 · Historical vehicle association; OBD-II compatibility is not established by this record.
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.
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 ·
- 01-19-05TT / 2054867/7 · No start or ECM communication, U0100 ↗
Volkswagen Group of America · PDF page(s) 1, 2 ·
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.