What does the U0100 fault code mean when the car won't start?
2 Answers
It is related to the loss of communication with the engine control module. Here is the relevant introduction: U0100 fault code: The U0100 fault code, applicable to all automobile manufacturers, means Lost Communication With Engine Control Module (ECM)/Powertrain Control Module (PCM) A. Engine Control Module The engine control module, also known as the engine control unit (in some vehicles, the engine control module and transmission control module are integrated together and referred to as the powertrain control module), controls the operation of the entire engine. Its most important function is to control the optimal fuel injection amount and ignition timing of the engine. Another important function is to manage components responsible for exhaust emission control.
The U0100 trouble code indicates a communication loss, primarily meaning the transmission control module (TCM) has lost connection with other control units. When this occurs, the vehicle may fail to start, a situation I frequently encounter during repairs. The main causes could be wiring issues such as loose connectors, broken wires, or short circuits, a faulty TCM module itself, a blown fuse, or even CAN bus malfunctions. For diagnosis, it's best to first check the fuses—locate and replace the corresponding numbered fuse in the engine compartment's fuse box. If that doesn't resolve the issue, use an OBD scanner to read specific data streams and identify where the connection is broken. Avoid forcing a start to prevent further damage to electronic components—safety first! A professional shop inspection is highly recommended for a comprehensive check.
