U0100 — Audi — A faulty PCM, A problem with the control module circuit, A problem with the CAN bus

The vehicle network (CAN bus, Controller Area Network) has stopped receiving signals from the engine control module (ECM) or powertrain control module (PCM). Another module, such as the instrument cluster or body control module, is trying to communicate with the ECM/PCM and receives no response.

Affected system: Network

Symptoms

• Check engine light permanently on
• Hard starting or no start
• Sudden loss of engine power, rough idle or limp mode
• Other dashboard warning lights flashing

Possible causes

• Power or ground issues: weak or dead 12 V battery, faulty alternator, loose or corroded ground straps
• Blown fuses protecting the ECM/PCM or the OBD2 port
• Damaged, chewed or water-corroded CAN bus wiring, or loose pins at the ECM harness connector (e.g. Audi T121 connector)
• Failed ECM/PCM due to internal hardware damage, water intrusion or a burnt circuit board (less common than wiring or power faults)

What to check

• Check the battery and charging system: the battery should hold a steady 12.3–12.7 V (higher with the engine running) and ground connections to the engine block must be tight
• Inspect fuses and relays in all fuse boxes (engine bay and cabin) for blown ECM/PCM fuses
• Test power and ground at the ECM harness connector with a multimeter: constant and switched 12 V supply pins, and ground resistance below 0.2 ohms
• Inspect wiring and harnesses for frayed wires, green corrosion on connector pins or moisture damage along the CAN bus lines
• If power, grounds and wiring are correct, have the ECM bench-tested, repaired or replaced