Lost Communication With ECM / PCM 'A'
Code U0100 indicates that one or more vehicle control modules (such as the transmission, ABS, or body controller) have completely lost CAN bus communication with the Engine Control Module (ECM/PCM). Primary causes include low battery voltage, blown main PCM fuse, or corroded ground lugs.
Do not condemn the computer first! 70% of U0100 codes are caused by weak battery terminals (< 12.0V), blown ECM/PCM power fuses, or corroded negative battery ground straps bolted to the transmission or frame.
Interactive Symptom Pinpoint Troubleshooter
Answer 2 diagnostic tests to isolate the exact component failure for U0100.
Does your multimeter measure 60 Ohms across OBD-II diagnostic pins 6 and 14 with the battery disconnected?
Live Scanner & Multimeter Benchmark Values
Compare your live OBD-II PID sensor data against OEM factory operating specs for U0100.
| Monitored Parameter / Sensor PID | Healthy OEM Operating Range | Fault Threshold (Triggers U0100) |
|---|---|---|
| Battery Resting Voltage | 12.4V – 12.8V | < 11.8V causes CAN bus transceivers to shut down |
| CAN High (CAN-H) Operating Voltage | 2.5V – 3.5V (nominal ~2.7V) | 0V indicates short to ground; 12V indicates short to battery |
| CAN Low (CAN-L) Operating Voltage | 1.5V – 2.5V (nominal ~2.3V) | Pinned voltage or open line |
| OBD-II DLC Pin 6 to Pin 14 Resistance | 58 – 62 Ohms (two 120Ω terminating resistors) | 120 Ohms indicates one terminating resistor severed; 0 Ohms is short |
Symptoms Associated with U0100
Drivers typically observe the following symptoms when the ECM flags U0100:
Common Root Causes & Statistical Probability Matrix
Statistical breakdown of what triggers this code based on automotive repair shop data.
Step-by-Step Diagnostic & Repair Procedure
Follow this logical order to isolate and repair U0100 before spending money on costly parts.
Step 1: Test Battery Voltage and Terminal Tightness
Battery resting voltage must be at least 12.4V. Clean terminal clamps until shiny lead is exposed.
Step 2: Measure CAN Bus Resistance at DLC Port
Disconnect negative battery cable. Measure Ohms between OBD-II DLC Pin 6 (CAN-H) and Pin 14 (CAN-L). Reading MUST be 58-62 Ohms. If 120 Ohms, one bus leg is severed.
Step 3: Check Engine Computer Ground Straps
Inspect woven copper ground straps from cylinder head to firewall. Voltage drop between battery negative and engine block must be < 0.05V.
Vehicle-Specific Failure Trends & Technical Bulletins
Real workshop failure patterns reported by mechanics across popular makes and models for U0100.
Chevy Silverado / Tahoe (2007-2018)
Ford F-150 / Explorer
Chrysler / Dodge / Jeep (TIPM)
Dave Miller, ASE Master Tech
ASE L1 Certified Master ReviewerASE Certified Master Automobile Technician (A1-A8) & L1 Advanced Engine Performance • 24 years automotive diagnostic experience
“Always verify sensor voltages and harness grounds before purchasing replacement electronic components.”