Code No.B120A Security alarm siren error <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.
  • When the ETACS-ECU of vehicles without KOS is replaced, the encrypted code of the ignition key needs to be registered to the ETACS-ECU. (If the encrypted code is not registered, the engine cannot be started. Register the encrypted code as described in Immobilizer System - How to Register Key ID .)


  • The security alarm siren communicates with the ETACS-ECU by the LIN. The diagnosis code No. B120A is set if the communication data between the ETACS-ECU and security alarm siren has an error.



STEP 1. M.U.T.-III diagnosis code.

Check if diagnosis code is set to the ETACS-ECU.

Q. Is the diagnosis code set?

Diagnose the ETACS-ECU. Refer to GROUP 54A, Diagnosis .
Go to Step 2.

STEP 2. Security alarm siren operation check

(1)Set the security alarm to the "Ready to be triggered" condition.
(2)Open the driver’s door by using the ignition key.
(3)Check that the security alarm siren sounds.

Q. Is the check result normal?

The trouble can be an intermittent malfunction (Refer to GROUP 00 - How to Cope with Intermittent Malfunction ).
Go to Step 3.

STEP 3. Replace the security alarm siren temporarily, and check whether the diagnosis code is set.

Check again if the diagnosis code is set to the ETACS-ECU.
(1)Erase the diagnosis code.
(2)Turn the ignition switch from the LOCK (OFF) position to the ON position.
(3)Check if diagnosis code is set.

Q. Is the diagnosis code set?

Replace the ETACS-ECU.
Replace the security alarm siren.