What Does U0100 Mean?
The requesting module cannot communicate with the engine control module (ECM/PCM). This is one of the most critical communication failures as the ECM controls engine operation.
Safety Warning
Vehicle may not start or may stall unexpectedly. Loss of ECM communication is a critical failure requiring immediate attention.
Common Causes
30%
Failed or failing engine control module (ECM/PCM)
25%
Open or shorted CAN bus wiring between ECM and other modules
20%
Blown fuse or loss of power to ECM module
15%
Corroded or damaged ECM connector terminals
10%
Poor ground connection at ECM causing communication failure
Diagnostic Steps
1
Step 1: Check all fuses related to ECM/PCM power supply and verify battery voltage is adequate (>12V)
2
Step 2: Inspect ECM connector for corrosion, bent pins, or backed-out terminals; clean and secure as needed
3
Step 3: Verify ground connections at ECM are clean, tight, and have continuity to battery negative
4
Step 4: Measure CAN bus voltage at ECM connector with ignition on (should read ~2.5V on each CAN line)
5
Step 5: Check for ECM software updates or known technical service bulletins; may require ECM replacement if all wiring checks pass
Estimated Repair Cost
$100 - $2,500
Parts + labor, varies by vehicle and location
The requesting module cannot communicate with the engine control module (ECM/PCM). This is one of the most critical communication failures as the ECM controls engine operation.
The most common cause of U0100 (Lost Communication With ECM/PCM "A") is: Failed or failing engine control module (ECM/PCM)
Typical repair costs for U0100 range from $100 to $2,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle may not start or may stall unexpectedly. Loss of ECM communication is a critical failure requiring immediate attention.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U0100 to identify the root cause.
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Quick Info
Category
Network
System
Engine Control
Difficulty
Type
Generic (SAE)
Recommended Tools
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