What this code means
The vehicle may be setting U128F because one blind spot radar unit may not be receiving expected communication from the other radar unit over the B-CAN network.
What the vehicle may do
- The blind spot information system may show a warning.
- The blind spot information system may be unavailable or behave inconsistently.
- The code can appear when the left side BSI radar unit may not start normally.
Possible fault areas
- Possible left side BSI radar unit power circuit issue.
- Possible left side BSI radar unit ground circuit issue.
- Possible B-CAN communication issue between the radar units.
Diagnostic path
Opening context
This one is for U128F on a 2016 to 2024 Honda Civic with the 2.0 gas engine. In plain language, the right side blind spot radar may not be hearing the left side blind spot radar over the B-CAN network. The driver may see a blind spot information warning or the system may act unavailable. Broadly, this can involve a left side radar unit that is not waking up normally, a power or ground issue to that unit, or a B-CAN communication problem. Start with the basic system checks before getting deep into the circuit testing, especially if the vehicle has other symptoms or communication-related codes present.
How the code sets
The logic behind U128F is straightforward: the right side BSI radar unit expects to receive signals from the left side BSI radar unit over B-CAN. If it cannot receive those signals, it treats that as an abnormal communication condition and stores the code. So do not start by guessing at a radar unit. First prove whether the left side unit is able to power up and communicate.
Primary diagnostic path
After the basic checks, go to the left side BSI radar unit and check its power circuit and ground circuit. That is the first direct test path for this code, because if the left side unit does not start normally, the right side unit may lose that B-CAN message and U128F can set. Keep the diagnosis clean: verify the feed and ground side before moving into network communication testing.
B-CAN direction
If the left side radar unit power and ground check does not explain the fault, continue the diagnosis as a B-CAN lost communication problem. At that point, the target is the communication path between the units, not a blind replacement decision. Pause, recheck the earlier diagnostic path, and avoid guessing if the return point in the communication diagnosis is not clear.
Confirmation
For confirmation, turn the vehicle to the ON mode, then wait for at least 5 seconds. Keep verification separate from the circuit checks: first complete the power, ground, and communication diagnosis, then use the ON-mode wait period to give the system a chance to run its check again.
Takeaway
The key on U128F is to follow the communication logic: the right side radar is missing the left side radar over B-CAN, so prove the left side unit can wake up, prove its power and ground, then move into the B-CAN lost communication path if needed. For more diagnostic training, visit stepdiagnostics.com.
Final check
U128F often points the diagnosis toward the left side radar unit power, ground, and B-CAN communication path before any repair decision is made.
For more guided automotive diagnostics, visit STEP Diagnostics.
