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 · Ambient Air Temperature (AAT) improvementsU0100Lost Communication With ECM/PCM)Manufacturer communications
11Original wording in English
RAM
Reported code forms U0100
Flash: Powertrain Diagnostic And System Improvements This bulletin involves reprogramming the Powertrain Control Module (PCM) with the latest available software. Customers may experience a Malfunction Indicator Lamp…
RAM
Reported code forms U0100
Flash: 6.7L Diagnostic And System Improvements This bulletin involves reprogramming the Engine Control Module/Powertrain Control Module (ECM/PCM) with the latest available software. A small number of customers may…
RAM
Reported code forms U0100
Flash: 6.7L Diagnostic And System Improvements This bulletin involves reprogramming the Engine Control Module/Powertrain Control Module (ECM/PCM) with the latest available software. A small number of customers may…
RAM
Reported code forms U0100
Flash: 6.7L Diagnostic and System Improvements This bulletin involves selectively erasing and reprogramming the Powertrain Control Module (PCM) with new software. Several software improvements are available for the…
RAM
Reported code forms U0100
Flash: 6.7L Diagnostic and System Improvements This bulletin involves selectively erasing and reprogramming the Powertrain Control Module (PCM) with new software. Several software improvements are available for the…
RAM
Reported code forms U0100
Flash: 6.7L Diagnostic and System Improvements This bulletin involves selectively erasing and reprogramming the Powertrain Control Module (PCM) with new software. Several software improvements are available for vehicles…
Check vehicle applicability
2A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
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.