Summary
U0100 means another control module (typically the BCM or gateway) has lost communication with the engine control module (ECM/PCM) over the CAN bus. The most common causes are CAN bus wiring faults (~30%), a failed ECM/PCM (~25%), and low battery voltage or poor grounds (~20%). This is a serious code — without ECM communication, the engine may not start, may stall, or may be stuck in a limp mode. Other modules lose access to critical engine data like RPM, coolant temperature, and throttle position.
Severity: High — engine management data is unavailable to other modules
Safe to drive: No — engine may stall, run poorly, or fail to start
Repair cost: $100–$1,500 depending on cause
DIY difficulty: Moderate to Hard
What does U0100 mean?
The ECM (Engine Control Module) or PCM (Powertrain Control Module) is the primary computer managing fuel injection, ignition timing, emissions, and many other engine functions. It broadcasts critical data — engine RPM, coolant temperature, throttle position, calculated load — on the CAN bus for other modules to use. The transmission module needs RPM data to shift, the instrument cluster needs it for the tachometer, and the ABS module needs it for traction control calculations.
U0100 is set when a requesting module expects to receive CAN messages from the ECM/PCM but those messages stop arriving for a defined timeout period (typically 2-5 seconds of missing messages). The code doesn't mean the ECM itself has set a fault — it means another module noticed the ECM went silent.
This can happen because the ECM itself has failed, because the CAN bus wiring between the ECM and the rest of the network is damaged, or because the ECM has lost power or ground and can't operate. On some vehicles, a failed ECM relay, blown fuse, or corroded ECM connector is the root cause rather than an expensive module failure.