What this code means
U128D may indicate that the BSI radar unit is not receiving expected gauge control module communication over the B-CAN network.
What the vehicle may do
- The vehicle may display warning messages.
- Driver-assist or communication-related functions can be affected.
- Symptoms may depend on whether the gauge control module starts normally and whether the B-CAN messages are present.
Possible fault areas
- Gauge control module power supply side
- Gauge control module ground side
- Possible module startup issue
- B-CAN communication path between the BSI radar unit and gauge control module
Diagnostic path
Opening context
For this 2016 to 2024 Honda Civic with the 2.0 gas engine, U128D is a network communication code. In plain terms, the BSI radar unit may not be getting the gauge control module messages it expects over the B-CAN network. The vehicle may have warning messages, driver-assist concerns, or communication-related symptoms, and the possible fault areas can include the gauge control module power and ground side, normal module startup, and the B-CAN communication path between the modules.
Set up the diagnostic approach
Before troubleshooting U128D, start with the basic troubleshooting approach first. Keep the setup and diagnostic direction consistent before you get into individual circuit checks, so you are not chasing a communication code without a clean starting point.
Start with module power and ground
Next, start at the gauge control module. Before chasing the network, check that the gauge control module power circuit and ground circuit are right. If that module does not start normally, this code may store, so do not treat the B-CAN message loss as a network failure until the module has a solid chance to wake up and operate.
Move into the B-CAN communication path
Once the gauge control module power and ground checks are handled, the diagnostic path moves into the B-CAN lost communication side. The logic is simple: if the BSI radar unit cannot receive the gauge control module signals through B-CAN, it judges that as abnormal and sets U128D. If you do not have the B-CAN lost communication test path available at this point, pause, recheck the earlier diagnostic path, and avoid guessing at the network or modules.
Verify the repair
Keep verification separate from the circuit testing. To confirm the code logic, turn the vehicle to the ON mode, then wait for at least 5 seconds. After repairs or corrections are made, use that same operating condition to verify the repair and confirm U128D stays gone.
Takeaway
The takeaway: use a structured starting point, prove the gauge control module can power up and ground correctly, then follow the B-CAN communication diagnosis instead of jumping straight to parts. For more diagnostic training, visit stepdiagnostics.com.
Final check
U128D is best approached by confirming gauge control module power and ground first, then continuing into the B-CAN communication diagnosis.
For more guided automotive diagnostics, visit STEP Diagnostics.
