U0100: Lost Communication With ECM/PCM "A"
Title: SAE J2012 generic definition. Manufacturer-specific definitions and enhanced codes can differ; check service information for the exact vehicle.
What U0100 means
Another module on the vehicle network stopped hearing from the engine computer (ECM/PCM).
What sets it
Modern vehicles connect modules on a network (usually CAN). Modules expect regular messages from each other. If a module such as the TCM, ABS or instrument cluster stops receiving the ECM's messages for a calibrated time, it stores U0100. The cause can be the ECM itself, its power or ground, or the network wiring between modules.
Symptoms
- Stall or no-start (if the ECM went offline)
- Several warning lights at once
- Gauges dropping out
- Scan tool can't connect to the ECM
Is it safe to drive?
General guidance, not a guarantee: Not recommended if the engine has stalled or cut out: a stall while driving is a safety risk. If it is a stored history code with no symptoms, diagnose before relying on the vehicle. If in doubt, have it towed and diagnosed.
Common causes
Listed roughly from most to least common in general practice. The real order varies by vehicle, mileage and history.
- ECM power or ground fault: blown fuse, corroded ground, low battery voltage
- Network wiring: damaged CAN-H/CAN-L wires, a connector or a splice pack
- Water intrusion at a module or connector
- ECM internal fault (after the above are ruled out)
- Aftermarket devices on the network or diagnostic port
Test before you replace: diagnostic sequence
General guidance only. Always follow the vehicle manufacturer's service information, specifications and safety procedures. Any figures below are general guidelines that defer to the manufacturer's specification.
- Check battery condition and charging voltage. Low system voltage can cause communication codes.
- See which modules report U0100 and whether the scan tool can reach the ECM at all.
- Check the ECM fuses, power feeds and grounds under load per the wiring diagram.
- Inspect connectors and grounds for corrosion and water, especially for modules mounted low or near the cowl.
- Check the CAN network at the diagnostic port or a splice. General guideline only: on many high-speed CAN buses with two 120-ohm terminating resistors, resistance between CAN-H and CAN-L with the battery disconnected reads about 60 ohms. Network designs vary, so use the manufacturer's wiring diagram and specification.
- Unplug aftermarket devices on the network and re-test.
- Condemn the ECM only after power, ground and network checks pass; a replacement may need programming.
Bench note: Most U0100 jobs come down to power, ground or wiring. Check those before you order a module.
How-to guides for this code
Related codes
FAQ
Does U0100 mean the ECM is bad?
Not necessarily. Power, ground, low battery voltage and network wiring are common causes. Rule them out first.
Why can't my scan tool connect?
If the ECM is offline, generic OBD tools can't talk to it. That is itself a useful clue.
Can a weak battery cause U-codes?
Yes. Low voltage during cranking can make modules drop off the network and store communication codes.
Sources
- SAE J2012, Diagnostic Trouble Code Definitions (standard page; code titles only)
- ISO 11898-2, Road vehicles: CAN high-speed medium access unit (standard page)
- Generic code title cross-check: obd-codes.com generic code list
Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information. Shop Docs Studio is not affiliated with SAE, EPA, CARB or any vehicle manufacturer.