U0100 — Lost Communication with ECM/PCM
Critical — Do Not DriveAt a Glance
What It Means
Communication has been lost between the body control module or other module and the Engine Control Module (ECM) / Powertrain Control Module (PCM) on the CAN bus network.
System
Network — CAN Bus
Can I Drive?
No — stop driving immediately.
Severity
Critical
Symptoms
- Multiple warning lights on dashboard
- Engine may not start
- Various systems inoperative
- Erratic gauge readings
Common Causes
How We Diagnose U0100
Scan & Verify
Confirm U0100 and check for related codes that may share a root cause.
Live Data Review
Pull real-time sensor data to isolate whether the fault is electrical, mechanical, or software-related.
Component Test
Perform targeted tests on the specific circuit or component flagged by the code.
Root Cause Repair
Repair or replace only what is confirmed faulty — no parts-swapping guesswork.
Verify & Clear
Clear the code, perform a drive cycle, and confirm the monitor passes before returning the vehicle.
Related Services
Frequently Asked
Will code U0100 clear itself?
OBD-II codes do not clear themselves. The underlying fault must be repaired and the monitor must pass during a drive cycle before the light extinguishes.
Can I pass an Arizona emissions test with U0100 active?
No. Any active check engine light is an automatic emissions test failure in Arizona. The code must be resolved and the relevant monitor must complete before testing.
How long does a diagnostic take?
Most diagnostic appointments at Apex take 45–90 minutes. Complex electrical faults may require additional time for circuit testing.