What this code means
U128F may indicate that the right-side BSI radar unit is not receiving the expected communication from the left-side BSI radar unit over the B-CAN network.
What the vehicle may do
- The blind spot information system may warn the driver or may not operate normally.
- The condition may appear during normal system wake-up or communication checks.
Possible fault areas
- Possible B-CAN communication trouble
- Possible left-side BSI radar unit startup issue
- Possible power or ground circuit issue for the left-side BSI radar unit
Diagnostic path
Code meaning and diagnostic direction
On this Civic, U128F is a communication fault in the blind spot information radar system. In plain terms, the right-side BSI radar unit may not be getting the signal it expects from the left-side BSI radar unit over the B-CAN network. The driver may see blind spot system warnings or the system may not operate normally. For diagnosis, think in broad areas first: possible B-CAN communication trouble, a left-side radar unit that may not be starting up correctly, or possible power and ground issues feeding that left-side unit.
Start with the basic checks
Before going into the code path, start with the basic system checks and follow a structured diagnostic approach. Do not jump straight to a radar unit decision. This code sets when the right-side radar unit cannot receive the left-side radar unit signal on B-CAN, so the communication path has to be treated as the main diagnostic direction.
Reproduce the monitor condition
To run the confirmation condition, turn the vehicle to the ON mode and wait for at least 5 seconds. That gives the BSI radar unit time to look for the expected communication from the opposite side.
Follow the communication fault path
If U128F is present after that check, follow the B-CAN lost communication diagnostic path. Also keep in mind that if the left-side BSI radar unit does not start normally, this DTC may store. As part of that direction, check the power circuit and ground circuit for the left-side BSI radar unit. If the path becomes unclear, pause, recheck the earlier diagnostic path, and avoid guessing.
Verify and close
Keep verification separate from the communication testing. After the fault path has been addressed, use the ON-mode confirmation check again and make sure U128F does not come back. The takeaway: this is a BSI radar communication code, so prove the B-CAN communication direction and the left-side radar unit power and ground before making any parts call. For more diagnostic training, visit stepdiagnostics.com.
Final check
U128F is best approached as a communication-path diagnosis, with attention on B-CAN communication and the left-side radar unit power and ground.
For more guided automotive diagnostics, visit STEP Diagnostics.
