What does U0100 lost communication with ECM mean?
Short answer: U0100 means another module stopped hearing from the engine computer on the network; the cause is usually power, ground or wiring to the ECM, or a network fault, rather than a failed module.
Modules on a CAN network constantly share messages. When the transmission computer, ABS module or instrument cluster expects messages from the engine computer and stops receiving them, it stores U0100. The code usually sets in the module that is listening, not the one that went quiet.
Start with the basics that kill communication: a weak battery, a loose or corroded ground, a blown fuse feeding the ECM, or a bad connection at the ECM connector. Check that the ECM has battery power, ignition power and good grounds under load, not just with a test light. Voltage drop testing on the grounds is worth the time.
Next, check the network itself. With the battery disconnected, measure resistance between CAN high and CAN low at the diagnostic connector, pins 6 and 14 on most vehicles. Around 60 ohms means both terminating resistors are present. Then look at the CAN signals with a scope. Only after power, ground and network tests pass should you consider the module itself, and if you replace one, it may need programming.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.