Code No.B120C Security alarm siren Flat Battery <Vehicles with security alarm sensor>

  • Before replacing the ECU or sensor, ensure that the power supply circuit, the earth circuit and the communication circuit are normal.


The diagnosis code No.B120C is set when the voltage of the rechargeable battery is low, or when the battery inside the security alarm siren has an error.
note The rechargeable battery integrated in the security alarm siren is charged by turning the ignition switch to the ON position.



M.U.T.-III diagnosis code.

Recheck whether the diagnosis code is set.
(1)Charge the rechargeable battery of the security alarm siren by turning the ignition switch to the ON position. (In principle, charge the battery for 30 minutes or more.)
(2)Erase the diagnosis code.
(3)After the diagnosis code is erased, turn the ignition switch to the LOCK (OFF) position.
(4)Activate the security alarm once, and then cancel it.
note Note that after erasing diagnosis code No.B120C, if the ignition switch is turned from the LOCK (OFF) position to the ON position before activating the security alarm, diagnosis code No.B120C will be set even though the system is normal.
(5)Turn the ignition switch from the LOCK (OFF) position to the ON position.
(6)On completion, check that the diagnosis code is not reset.

Q. Is the diagnosis code set?

Replace the security alarm siren.
The diagnosis is complete.