U0100: Lost Communication With ECM/PCM "A"

What U0100 means

U means network (communication between modules). The 0 after it marks a generic code defined by SAE rather than by the manufacturer, and the U01xx block is reserved for lost-communication faults with specific modules.

Modern vehicles connect their control modules on a shared data network, most often CAN bus. Each module expects regular messages from the others. A "lost communication" code is set by a module that stopped receiving expected messages from the named module for longer than an allowed time.

The module named here is the engine (and often transmission) control module. What you notice depends on what that module does: engine running problems, a no-start, or the module not responding to a scan tool at all.

What “Lost Communication” means here

A module on the network expected periodic messages from the engine (and often transmission) control module and did not receive them for longer than allowed. The code is stored in the module that noticed the silence, not necessarily in the one that went quiet, and the named module may itself be unpowered, faulty, or cut off by wiring.

What you may notice

Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany U0100:

• Warning lights for the system the missing module controls
• Features of that system not working
• Several U-codes stored at once in different modules

Causes to consider

• Blown ECM main power fuse
• Corroded CAN bus wire
• Dead 12V battery

Where to start diagnosing U0100

1. Check battery voltage and the charging system first; low voltage causes many communication faults.
2. Check whether the named module is powered: its fuses, power feeds and grounds.
3. Scan all modules: if one module is silent to every other module, focus on it; if many modules are missing, suspect the network wiring.
4. Inspect network wiring and connectors near the named module; on CAN, the two termination resistors should measure about 60 Ω together across the bus pins with the battery disconnected.

Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.

Live data to watch

These standard OBD2 values relate to U0100. Support varies by vehicle; a value your car does not report will simply be missing.

Related codes

Codes from the same vehicle network communication family. If several are stored together, the shared network wiring, power supply or ground is the place to start.

Further reading

• CAN bus architecture and network faults

Symptoms that lead to U0100

If you have not scanned the car yet, or U0100 is one of several codes stored, these guides start from what the vehicle is actually doing.

• It cranks, but it will not fire The starter turns the engine over normally and nothing catches. That narrows things down quickly: the engine is…
• Several warning lights came on together When unrelated systems misbehave simultaneously — ABS, airbag, engine, transmission — the usual cause is not several…

Frequently Asked Questions