What this code means
B1129 may indicate that the MICU is seeing a driver's door lock knob switch input problem where lock and unlock signals can appear active together.
What the vehicle may do
- The power door locks may behave inconsistently.
- The driver's door lock status may be reported incorrectly.
- The security system may react unexpectedly if door lock status information is wrong.
Possible fault areas
- Possible driver's door latch or knob switch fault.
- Possible shorted lock or unlock signal wiring.
- Possible MICU or under-dash fuse and relay box fault area.
Diagnostic path
Code overview
On this 2010 to 2017 Honda Accord with the 2.4 gas engine, B1129 is a driver's door knob switch input circuit malfunction. In plain terms, the MICU is seeing a condition where the lock and unlock inputs can appear active at the same time. The vehicle may show incorrect driver's door lock status, and the power lock or security behavior may be inconsistent. The possible fault areas here are the driver's door latch and knob switch, the lock and unlock signal wiring, and the MICU area in the under-dash fuse and relay box. The diagnostic path is to prove whether the signal is coming from the latch, being pulled down in the wiring, or being interpreted incorrectly by the MICU.
Check the switch inputs with the latch connected
Start with the driver's door lock knob switch in the lock position. Using the HDS, look at the driver's door lock knob switch LOCK input and the driver's door lock knob switch UNLOCK input. The key question is whether both information indicators are ON at the same time. If both are ON, keep going to isolate where that simultaneous input is coming from. If they are not both ON, treat it as an intermittent failure; the system is OK at this time.
Separate the latch from the circuit
Next, turn the ignition switch to LOCK (0), or press the engine start/stop button to select the OFF mode. Disconnect the driver's door latch 10P connector. Then check the same two HDS inputs again: driver's door lock knob switch LOCK and driver's door lock knob switch UNLOCK. If either information indicator is still ON with the latch disconnected, move on to the wiring short check. If neither indicator is ON, the failure points to the driver's door lock knob switch, and the supported repair is to replace the driver's door latch.
Check the lock and unlock signal wires for a short to ground
For the wiring check, keep the ignition switch at LOCK (0), or the engine start/stop system in OFF mode. Leave the driver's door latch 10P connector disconnected, and disconnect under-dash fuse/relay box connector P (24P). Now check continuity to body ground on the two circuits, one at a time. For DR SILCON UNLOCK, test from under-dash fuse/relay box connector P (24P) No. 10 (LT BLU) to body ground. For DR SILCON LOCK, test from under-dash fuse/relay box connector P (24P) No. 9 (BLU) to body ground. If there is continuity, repair the short to ground in the affected wire. If there is no continuity, the DR SILCON UNLOCK and DR SILCON LOCK wires are OK; the supported conclusion is a faulty MICU, and the supported repair is to replace the under-dash fuse/relay box.
Takeaway
The key on B1129 is not to guess at the latch or the MICU. First confirm the simultaneous LOCK and UNLOCK input, then unplug the latch to see if the input disappears, and only then check the two signal wires for a short to ground. That sequence tells you whether the problem is in the driver's door latch, the wiring, or the MICU side of the circuit. For more diagnostic training, visit stepdiagnostics.com.
Final check
This diagnosis often comes down to separating the latch input from the wiring and MICU side before making a repair decision.
For more guided automotive diagnostics, visit STEP Diagnostics.
