U0100
Lost Communication With ECM/PCM A
Network / Communication Network/Communication CAN Bus Communication Failure 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

Your vehicle has lost the communication link between the main engine computer (ECM) and the transmission control module or other critical control unit—like a phone losing signal to its tower. Without this connection, the vehicle cannot coordinate engine and transmission operation properly.

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Symptoms You May Notice
3 known symptoms for this code
Engine starts but runs roughly or stalls frequently
Transmission does not shift or shifts erratically
Multiple warning lights illuminate on dashboard
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECM expects to receive periodic CAN bus messages from the PCM/TCM within a specific time window. If no valid message arrives within 1-2 seconds, the ECM sets a timeout fault. The network monitors signal voltage, message integrity, and handshake responses.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
CAN Bus Message Timeout Message received every 100-500 ms No valid message for >1000 ms
CAN Bus Voltage 2.5–4.5 V differential <0.5 V or shorted to ground
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
OBD-II scanner
Clear the fault code and retest to confirm if the error is intermittent or persistent.
2
Battery terminals and ground cables
Clean corrosion from battery terminals and inspect all ground connections between engine block and chassis for loose or corroded terminals.
3
CAN bus wiring and connectors
Inspect the CAN bus harness under the dash and engine bay for damaged insulation, unplugged connectors, or chewed wires; reconnect or tape as needed.