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.

Real-world scan data

Seen in

13,383

scans in the past year

Found in

2.1%

of scans

3.8%

of scans that found faults

Appears alongside · share of U0100 scans

U0121in 26.9% of scans

C1057in 25.4% of scans

U0151in 16.9% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear U0100 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX