U0100
Lost Communication With ECM/PCM A
If your vehicle's onboard computer has flagged the diagnostic trouble code U0100, it refers to a detected anomaly regarding "Lost Communication With ECM/PCM A". This systemic engine fault needs a targeted check before symptoms expand.
Driver's Summary
Code U0100 means your vehicle detected a problem with the lost communication with ecm/pcm a system. Typical symptoms include engine dies while driving, cranks but won't start, gauges drop to zero. Given the high severity of this code, continuing to drive risks significant mechanical damage. Have it diagnosed immediately.
Symptoms
Engine dies while driving, cranks but won't start, gauges drop to zero
Common Causes
- Failed PCM relay or blown fuse
- Corroded ground to PCM
- Broken CAN bus wire
- Dead PCM
How to Fix
- 1 Check PCM power and ground circuits
- 2 Replace PCM relay/fuses
- 3 Perform CAN bus continuity test
- 4 Replace and program new PCM
Technical Explanation
Code U0100 is confirmed when the ECM's diagnostic algorithm detects a parameter deviation that persists across a defined number of consecutive drive cycles. The affected module's power supply circuit (fuse, relay, and ground) is the first area the PCM checks via its own power monitoring circuits before flagging the loss of communication as a module fault rather than a network fault. The fault remains stored in memory even after the MIL is cleared; it becomes a confirmed DTC after failing two consecutive drive cycles, and the PCM logs a freeze frame record of the engine's exact operating state at the moment of detection.
Is It Safe to Drive?
Driving with an active U0100 fault risks accelerating damage to failed pcm relay or blown fuse and related components. The longer the fault persists, the more expensive the eventual repair becomes — what starts as a sensor or solenoid issue can escalate to major mechanical failure.
Mechanic's Pro Tip
Module replacement should always be the last resort for U0100 after exhaustively verifying all power supply circuits, ground connections, and communication bus wiring. Use a wiring diagram to locate all fuses, relays, and ground points for the affected module, and measure voltage drop on each ground with the circuit loaded. A module "failure" is frequently a corroded ground eyelet or a weak battery causing brownout conditions — fix these first and you'll save hundreds of dollars on an unnecessary module replacement.
Relay/Fuse: $20; PCM Replacement/Programming: $600 - $1,200