U0100 — Lost Communication With ECM/PCM A
U0100 is reported by other modules that can no longer hear the engine module. It is usually a wiring, connector or power supply fault on the network rather than a failed module.
Behaviour varies from warning lights to a vehicle that will not start. Systems that depend on network messages, including stability control and transmission shifting, may be disabled. This code will cause an emissions test failure.
What U0100 means
Modern vehicles connect their modules on a CAN bus, and each expects regular messages from the others. U0100 means at least one module stopped receiving them from the engine control module. That can mean the engine module lost power or ground, that the bus itself is damaged, or — least often — that the module has failed. Because low system voltage disrupts communication generally, this code frequently appears alongside charging faults and clears once the supply is fixed.
| System | Network — module communication |
|---|
Symptoms you may notice
What causes U0100
Engine module has lost power or ground
A blown fuse, a failed relay or a corroded ground stops the module transmitting. It is the most common cause and the cheapest to check, and it explains why the module appears dead when it is not.
CAN bus wiring damaged
The two bus wires are twisted together and depend on that pairing. A chafe, a rodent bite or damage from previous work breaks communication for every module downstream.
Low system voltage disrupting the bus
Modules stop communicating reliably when supply voltage falls. If charging codes are stored alongside, fix the voltage before diagnosing the network at all.
Common misdiagnosis
Never diagnose network codes on a vehicle with a charging fault. Repair the supply first and re-scan — many of the codes will have gone.
Engine control module failed
Genuine module failure is uncommon and diagnosed last. Replacement usually requires programming to the vehicle, which makes guessing here expensive.
How to diagnose it
Check battery voltage and charging codes first
Measure system voltage and scan for charging faults. Low voltage disrupts the bus and produces network codes that vanish once the supply is repaired.
Try to communicate with the engine module directly
Attempt to read live data from the engine module. Whether it answers at all separates a module with no power from a bus fault affecting only some modules.
Check module power and ground
Confirm battery voltage on the module supply pins and a clean ground, referring to the wiring diagram for pin identification.
Measure resistance across the bus
With the ignition off, measure across the two CAN pins at the diagnostic connector. A healthy bus with both terminating resistors reads about 60 ohms; roughly 120 indicates one branch is open.
Related codes
Common questions
Does U0100 mean my computer has failed?
Rarely. It means other modules cannot hear it, which is far more often caused by a fuse, a ground, damaged bus wiring or low voltage than by the module itself.
Why are so many warning lights on?
Systems that depend on messages from the engine module cannot function without them, so each reports its own fault. They usually all clear once communication is restored.
Can I diagnose this with a basic scanner?
Only partly. Network faults need a scanner that can address individual modules to establish which are answering and which are not.
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.