OBD2 Code U0100: Lost Communication with ECM/PCM
The U0100 code means Lost Communication with ECM/PCM. Estimated repair cost: $200 – $1500.
⛔ STOP IMMEDIATELY if Check Engine Light is flashing. If steady: drive gently to a repair shop within 1-2 days.
📋 Diagnostic Reference
| OBD2 Code | U0100 |
| Category | Network |
| Severity | Check Soon |
| Repair Cost | $200 – $1500 |
| Can You Drive? | ⛔ STOP NOW if flashing / ⚠️ Diagnose soon if steady |
🔍 Symptoms
- Check Engine Light on
- multiple warning lights
- engine may not start or run rough
- transmission may default to limp mode
⚠️ Most Likely Causes (in order of probability)
- Failed ECM/PCM
- damaged CAN bus wiring
- loose ECM connector
- low battery voltage
- water damage to ECM
🛠️ Diagnostic Procedure
Step 1: Check battery voltage — low voltage causes communication errors
Check battery voltage — low voltage causes communication errors
Step 2: Inspect ECM connectors for corrosion or water
Inspect ECM connectors for corrosion or water
Step 3: Check CAN bus wiring for damage
Check CAN bus wiring for damage
Step 4: Scan all modules — if only one module lost communication the CAN bus is likely f...
Scan all modules — if only one module lost communication the CAN bus is likely fine.
ASE Certified Review: This diagnostic guide has been reviewed by an ASE-certified master technician. Diagnostic steps follow industry best practices per SAE J1979 and OEM service procedures. Always verify with a qualified mechanic before performing repairs.
Technician's Notes: Communication codes are often caused by low battery voltage. Charge or replace the battery first before replacing expensive modules.
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Data Sources: Diagnostic information compiled from SAE J1979 OBD2 standards, OEM service manuals, and ASE master technician field experience. Reviewed for technical accuracy. Repair cost ranges are national estimates and vary by location. Always verify with a qualified mechanic.