Network communication (U-codes) · Network · generic (SAE-defined)
U0100: Lost Communication With ECM/PCM "A"
U0100 means one or more control modules stopped receiving messages from the engine or powertrain control module (ECM/PCM) over the vehicle network.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How U0100 sets
Modules on the controller area network (CAN) expect regular messages from the ECM or PCM. If those messages stop arriving for longer than an allowed period, the listening module records U0100. The time window and enabling conditions are manufacturer-specific.
Common causes of U0100
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Low battery voltage or weak charging system
- Loose or corroded ECM/PCM connector
- Blown fuse or failed relay feeding the ECM/PCM
- Poor ECM/PCM ground
- Damaged CAN wiring to the ECM/PCM
- Failed ECM/PCM
- Water intrusion into the module
U0100 symptoms
- Check engine light and other warning lights on
- No-start or stalling
- Gauges such as the tachometer not working
- Transmission shifting harshly or stuck in one gear
How U0100 is diagnosed
- Scan all modules and see whether the ECM/PCM itself communicates with the scan tool.
- Test battery and charging voltage, and inspect battery terminals.
- Check ECM/PCM power and ignition fuses and relays.
- Verify ECM/PCM grounds with a voltage drop test.
- Check CAN high and low wiring continuity between modules.
- Inspect the module connector for corrosion or water damage.
Typical fixes for U0100
- Repair battery, terminals, or charging faults
- Replace blown fuses and repair the cause
- Repair ECM/PCM power or ground circuits
- Repair CAN wiring or connectors
- Replace and program the ECM/PCM if confirmed faulty
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
U0100 FAQ
What does code U0100 mean?
U0100 means one or more control modules stopped receiving messages from the engine or powertrain control module (ECM/PCM) over the vehicle network. The standard definition is "Lost Communication With ECM/PCM "A"."
Can I drive with a U0100 code?
It depends on the cause, but caution is warranted. The engine may stall, fail to start, or run with the transmission in a default mode, so have it diagnosed before relying on the vehicle.
Can U0100 be caused by a dead battery?
Yes. A weak battery or a jump-start event can cause temporary communication loss and store U0100 in several modules.
Does U0100 mean the engine computer is bad?
Not necessarily. Power, ground, and wiring faults are more common, so they should be ruled out before replacing the module.
Keep researching
More network communication (u-codes) codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.