What this code means
U128D means the auto high-beam control unit may have lost communication with the gauge control module on the body communication network.
What the vehicle may do
- The auto high-beam function may not operate normally.
- A lighting-related warning or message may appear.
- The vehicle may otherwise seem to drive normally.
Possible fault areas
- Possible gauge control module power or ground problem.
- Possible fuse or feed circuit problem.
- Possible B-CAN communication line open or poor connection.
- Possible loose or poor connector contact.
Diagnostic path
Opening context
On this Accord, U128D means the auto high-beam control unit has lost communication with the gauge control module. In plain terms, the auto high-beam system may not be hearing from the module it needs on the body communication network. The driver may notice the auto high-beam function not working, a lighting-related message, or the rest of the vehicle may seem mostly normal. Broadly, this can involve gauge control module power and ground, the B-CAN communication lines, fuse feeds, or loose and poor connector contact. If other body network codes are present, do not chase this code in isolation; handle the B-CAN multiple-code checks first.
Verify the code first
Start by proving the fault is active. Clear the DTCs with the HDS. Press the engine start/stop button to OFF, then back ON. Wait for at least 6 seconds, then check DTCs again with the HDS. If U128D comes right back, continue into the circuit checks. If U128D does not return, treat it as an intermittent failure for now; the system is OK at this time, and the next move is checking for loose or poor connections.
Decide which side of the network path to test
Next, use the HDS body electrical menu to look at the B-CAN connected control units information. Check whether the gauge control module shows as DETECT. If the gauge control module is detected on the network, skip ahead to the B-CAN line check between the auto high-beam control unit and the gauge control module. If the gauge control module is not detected, stay with the gauge control module power, fuse, ground, and network feed path first.
If the gauge control module is not detected, check its fuses
When the gauge control module is not detected, check fuse No. A21, rated 10 A, in the under-hood fuse/relay box, and fuse No. B5, rated 7.5 A, in the under-dash fuse/relay box. If the fuses are OK, keep moving down the power and ground checks. If a fuse is not OK, replace the fuse. If it blows again, repair a short to ground in the No. A21 (10 A) and/or No. B5 (7.5 A) fuse circuit.
Check the backup power feed
With the start/stop button in OFF mode, disconnect gauge control module connector A, the 32P connector. Measure voltage on the +B BACK UP circuit from connector A terminal No. 1, the WHT wire, to body ground. If there is battery voltage, that backup feed is OK and you move to the ignition feed. If there is not battery voltage, repair an open or high resistance in that wire.
Check the ignition meter feed
Now press the start/stop button to ON mode and leave the gauge control module connector A disconnected. Measure the IG1 METER circuit from connector A terminal No. 17, the BRN wire, to body ground. If there is battery voltage, the IG1 METER wire is OK and the next check is ground. If there is not battery voltage, repair an open or high resistance in that wire.
Check the gauge control module grounds
Switch back to OFF mode. With gauge control module connector A still disconnected, check continuity to body ground on the ground circuits individually: connector A terminal No. 4, BRN, and connector A terminal No. 32, BRN. If both have continuity, the ground wires are OK and you move to the B-CAN line check. If continuity is missing, repair an open or high resistance in the ground wire, or a poor ground at G502.
B-CAN line check when the gauge module was not detected
For the not-detected path, disconnect the auto high-beam control unit 6P connector. With the system in OFF mode, the gauge control module connector A disconnected, and the auto high-beam control unit connector disconnected, check continuity on the B-CAN_H circuit between the auto high-beam control unit 6P connector No. 2, PNK, and gauge control module connector A No. 30, PNK. If there is continuity, the diagnostic result points to an open or poor connection in the B-CAN_L wire between the auto high-beam control unit and the gauge control module. If there is not continuity, it points to an open or poor connection in the B-CAN_H wire between those modules.
B-CAN line check when the gauge module was detected
If the gauge control module did show DETECT earlier, the path goes directly to the B-CAN line check. Put the vehicle in OFF mode, disconnect gauge control module connector A, the 32P connector, and disconnect the auto high-beam control unit connector. Before probing this branch, pause and verify the exact auto high-beam control unit connector callout at the vehicle, because the connector designation for this test point is not consistent. The circuit logic is still the same: check continuity on B CAN-H between the auto high-beam control unit No. 2 PNK terminal and gauge control module connector A No. 30 PNK. If continuity is present, the result points to an open or poor connection in B-CAN_L. If continuity is not present, the result points to an open or poor connection in B-CAN_H.
Final confirmation
After correcting the fuse, feed, ground, or B-CAN wiring issue found by the path, verify it the same way you proved the fault. Clear the DTCs with the HDS, cycle OFF and ON, wait for at least 6 seconds, and confirm U128D does not return. The takeaway is simple: prove the code first, use the gauge module DETECT status to choose the branch, then separate power, ground, and B-CAN line faults without guessing. For more diagnostic training, visit stepdiagnostics.com.
Final check
This code is often approached by confirming the fault, checking whether the gauge control module is detected on B-CAN, then testing the related power, ground, and communication circuits.
For more guided automotive diagnostics, visit STEP Diagnostics.
