What this code means
U0100 may indicate that a control unit has lost communication with the ECM or PCM over the vehicle communication network.
What the vehicle may do
- The vehicle may show keyless access or engine start-stop related symptoms.
- Warning indicators or communication-related scan tool messages may be present.
- The concern can be intermittent, especially when a connection is loose or poor.
Possible fault areas
- Possible F-CAN communication wiring concern.
- Possible loose or poor connector connection.
- Possible keyless access power control unit communication concern.
- Possible ECM or PCM side communication path concern.
Diagnostic path
Opening context
On this 2010 to 2015 Honda Civic with the 1.8 gas engine, U0100 means the keyless access power control unit is not communicating with the ECM or PCM the way it should. The car may have keyless access or start-stop related complaints, and the fault can involve the F-CAN communication wiring, connector fit, terminal condition, or a possible control unit communication issue. The key is to prove whether the code is current first, then check the communication lines before making a call on the module.
Confirm the fault is active
Start by clearing the DTCs with the HDS. Then press the engine start-stop button to select OFF mode, and then ON mode. After that, wait for at least 6 seconds, and check for DTCs again with the HDS. If U0100 is not indicated, treat it as an intermittent failure. The system is OK at this time, so check for loose or poor connections. If U0100 is indicated again, continue into the F-CAN open-wire check.
Set up the F-CAN open-wire check
With the fault confirmed, press the engine start-stop button to select OFF mode. Jump the SCS line with the HDS. Then disconnect the correct engine control connector for the transmission: on a manual transmission, disconnect ECM connector A, 49P; on a CVT, disconnect PCM connector A, 51P. Also disconnect keyless access control unit connector C, 24P.
Check continuity on the F-CAN lines
Now check continuity between the keyless access control unit and the ECM or PCM. On a manual transmission, with ECM connector A, 49P disconnected and keyless access control unit connector C, 24P disconnected, check F-CAN-H between keyless connector C No. 4, WHT, and ECM connector A No. 3, WHT. Then check F-CAN-L between keyless connector C No. 5, RED, and ECM connector A No. 4, RED. On a CVT, with PCM connector A, 51P disconnected and keyless access control unit connector C, 24P disconnected, check F-CAN-H between keyless connector C No. 4, WHT, and PCM connector A No. 3, WHT. Then check F-CAN-L between keyless connector C No. 5, RED, and PCM connector A No. 4, RED. If continuity is present, the F-CAN wires are OK, and the procedure calls for replacing the keyless access control unit. If continuity is not present, the fault is an open or poor connection in the F-CAN wires between the ECM or PCM and the keyless access control unit.
Takeaway
The important part of U0100 on this system is not to guess at the module first. Confirm the code comes back, separate an intermittent connector issue from a current fault, and then prove the F-CAN circuit continuity before making the final call. For more diagnostic training, visit stepdiagnostics.com.
Final check
Confirm whether the code is current, then test the communication path before deciding where the fault is.
For more guided automotive diagnostics, visit STEP Diagnostics.
