U0100
Lost Communication With ECM/PCM "A"
Communication network
Generic OBD-II
High
What it means
U0100 means a module on the car's network has lost communication with the engine control module (ECM/PCM). When modules can't reach the ECM over the CAN bus, systems that rely on engine data stop working — so the car often runs badly or won't start.
High
Can I keep driving? Often no — U0100 frequently comes with a no-start or limp mode. Get it diagnosed before relying on the vehicle.
Common symptoms
What you'll notice
- Check-engine light, often with several other warning lights
- No-start or intermittent stalling
- Limp mode / severely reduced power
- Gauges or dash readouts behaving erratically
Common causes
Most likely first
- ECM power or ground fault (blown fuse, corroded ground)Likely
- Damaged CAN bus wiring or connectorsCommon
- Low battery voltage or a charging-system faultCommon
- Corrosion at module connectorsLess often
- Failed ECM/PCM (least common — rule out the above first)Less often
How to diagnose & fix
Work through it in order
- Check battery voltage, then the ECM fuses and grounds — the most common root cause.
- Scan all modules and note which report U0100 to narrow the affected bus segment.
- Inspect CAN wiring and connectors for damage, chafing, or corrosion.
- Test ECM power and ground under load.
- If power, grounds, and wiring are all good, suspect the ECM. Repair and re-test.
FAQ
Quick answers
Will the car start with U0100?+
Often not — it commonly causes a no-start because modules can't reach the ECM.
Does U0100 mean the ECM is bad?+
Not necessarily. Check power, grounds, and wiring first — a bad ECM is the least common cause.
Related codes
Often seen with U0100
Read & clear U0100 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX