U0100 fault code: lost communication with the ECM
Plain-language definition
Lost Communication With ECM/PCM A
What does it mean?
In modern cars, modules such as the engine computer, transmission, ABS and instrument cluster constantly send each other messages over a data network (usually CAN). U0100 means the module that stored the code hasn't received the messages it expects from the engine computer (ECM/PCM) for a set time. That's why the code is usually found not in the engine computer but in the memory of another module, such as the transmission, ABS or instrument cluster.
The record may be left over from a brief loss of communication when the battery was weak or the voltage dropped; during a battery replacement or a jump-start, many modules can store codes like this at the same time. If the code shows up again and again in more than one module, the problem may be in the engine computer's power supply, its connections or the data network.
The symptoms depend on how long communication was lost: you may notice the engine not starting or stalling, many warning lights coming on at once on the instrument cluster, the rev counter or temperature gauge not working and the transmission switching to limp mode. If it's a past record, there may be no symptoms at all.
How serious is it?
It's rated Warning; sometimes it's a one-time record left behind during a voltage drop. But if the code keeps coming back, or there are symptoms such as the engine stalling, the engine computer's power supply and data network connections need to be checked.
Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.
Common general causes
- Low battery voltage; a temporary record left behind during a battery replacement or jump-start
- A problem with the engine computer's supply fuse, relay or ground connection
- A loose connection, corrosion or water getting into the engine computer's connector
- Broken wiring, a short circuit or chafing in the data network (CAN) wiring
- A fault in another module on the data network affecting the whole network
- Less often, a fault in the engine computer itself
The list shows general causes; a workshop finds the actual cause on your car with a check.
Definitions by make
Some manufacturers record the same code under their own wording:
- GM
- Lost Communication With Engine Control Module Invalid Data; Lost Communication With Engine Control Module Message Counter Incorrect; Lost Communication With Engine Control Module Bus Signal Checksum Error
Related codes
How does it appear in the 2dakika report?
Lost Communication With ECM/PCM A
If the code is read as a confirmed (stored) fault, the report shows it with the "Warning" severity; in pending or past records, codes that don't request the warning light and aren't critical appear as "Info". If the engine computer stored them, the conditions when the fault was recorded (engine temperature, rpm, speed) are shown too.
Read this code with the OBD-X1, and clear it from the app after the repair.
Clearing the code doesn't fix the fault; if the problem persists, the code comes back.