What it means
A control module has lost communication with ECM/PCM A. The code does not establish why communication was lost or whether the engine will start; those effects depend on the vehicle and which module reported the code.
How it works
The ECM/PCM is the engine or powertrain control module, and other modules exchange data with it over the vehicle network. U0100 sets in a reporting module when expected messages from ECM/PCM A are missing for a defined period; it identifies lost communication, not the reason for it.
Common causes
Most likely first.
- Loss of ECM/PCM power or ground
- Open, shorted, or damaged communication wiring or connector
- ECM/PCM or gateway fault
Symptoms
- Communication faults in other modules
- Warning lights or messages
- Engine starting or running problems may occur
How to diagnose it
- Scan all modules and note which ones report U0100, whether it is current, and any accompanying network or power codes.
- Check battery condition and system voltage, then inspect the ECM/PCM fuses and grounds because loss of module power or ground can silence it.
- Inspect accessible ECM/PCM connectors and network wiring for looseness, corrosion, water intrusion, or damage.
- Use the wiring diagram to check the ECM/PCM power and ground circuits under the manufacturer's specified conditions.
- If power and ground are correct, test the network wiring and communication at the ECM/PCM and gateway as specified before suspecting a module fault.
Common fixes
- Charge or replace a weak battery and correct any charging or connection problem.
- Repair a failed ECM/PCM fuse, power feed, or ground connection.
- Repair damaged network wiring or clean and secure affected connectors.
- Repair or replace a gateway or ECM/PCM only after circuit and configuration checks confirm the fault.
Can I keep driving?
Whether it is safe to continue depends on whether the engine runs normally and whether essential warnings or functions are lost. If it stalls, will not restart, or shows a serious powertrain warning, stop in a safe place; continued driving could leave you without reliable engine control.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.