Code No.U0121 ABS/ASC CAN timeout
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- If diagnosis code U0121 is set in the transfer-ECU, always diagnose the CAN main bus line.
- Whenever the ECU is replaced, ensure that the communication circuit is normal.
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DIAGNOSIS CODE SET CONDITIONS
Current trouble
- Connector(s) or wiring harness in the CAN bus lines between the ABS-ECU <Vehicles without ASTC> or ASTC-ECU <Vehicles with ASTC> and the transfer-ECU, the power supply system to the ABS-ECU or ASTC-ECU, the ABS-ECU or ASTC-ECU itself, or the transfer-ECU may be defective.
Past trouble
- Carry out diagnosis with particular emphasis on connector(s) or wiring harness in the CAN bus lines between the ABS-ECU or ASTC-ECU and the transfer-ECU, and the power supply system to the ABS-ECU or ASTC-ECU. For diagnosis procedures, refer to "How to cope with past trouble" (Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points
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For a past trouble, you may not find it by the M.U.T.-III CAN bus diagnostics even if there is any failure in CAN bus lines. In this case, refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points . and check the CAN bus lines. You can narrow down the possible cause of the trouble by referring to the diagnosis code, which is set regarding the CAN communication-linked ECUs (Refer to GROUP 54C - CAN Bus Line Diagnostic Flow ).
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PROBABLE CAUSES
- Damaged wiring harness or connector
- Malfunction of the transfer-ECU
- Malfunction of the ABS-ECU
- Malfunction of the ASTC-ECU
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STEP 1. M.U.T.-III CAN bus diagnostics
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Using M.U.T.-III, perform CAN bus line diagnosis.
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Q.
Is the check result normal?
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 Go to Step 2.
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 Repair the CAN bus line (Refer to GROUP 54C - Troubleshooting ). and then go to Step 6.
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STEP 2. M.U.T.-III other system diagnosis codes
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Check whether the ABS-ECU or ASTC-ECU diagnosis codes are set or not.
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Q.
Is the diagnosis code set?
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YES : Diagnose the ABS-ECU (Refer to GROUP 35B - Troubleshooting ) or ASTC-ECU (Refer to GROUP 35C - Troubleshooting ).
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Go to Step 3.
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STEP 3. Check the M.U.T.-III Diagnosis code for other systems
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Is diagnosis code No. U0121 set in the other ECU, which communicates with the ABS-ECU or ASTC-ECU via CAN?
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Q.
Is the diagnosis code set?
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Go to Step 4.
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Go to Step 5.
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STEP 4. Check whether the diagnosis code is set again.
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Check again if the diagnosis code is set in the transfer-ECU.
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(1)Erase the diagnosis code.
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(2)Turn the ignition switch from "LOCK" (OFF) position to "ON" position.
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(3)On completion, check that the diagnosis code is set again.
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Q.
Is the diagnosis code set?
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Replace the ABS-ECU (Refer to GROUP 35B - Hydraulic unit ) or ASTC-ECU (Refer to GROUP 35C - Hydraulic unit ). After replacing the ECU, carry out the CAN bus diagnostics. Then go to Step 6.
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A poor connection, open circuit or other intermittent malfunction is present in the CAN bus lines between the ABS-ECU or ASTC-ECU and the transfer-ECU (Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).
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STEP 5. Check whether the diagnosis code is set again.
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Check again if the diagnosis code is set in the transfer-ECU.
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(1)Erase the diagnosis code.
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(2)Turn the ignition switch from "LOCK" (OFF) position to "ON" position.
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(3)On completion, check that the diagnosis code is not set again.
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Q.
Is the diagnosis code set?
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Replace the transfer-ECU (Refer to ). After replacing the ECU, carry out the CAN bus diagnostics. Then go to Step 6.
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A poor connection, open circuit or other intermittent malfunction is present in the CAN bus lines between the ABS-ECU or ASTC-ECU and the transfer-ECU (Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).
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STEP 6. Check whether the diagnosis code is set again.
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Recheck if the diagnosis code is set in the transfer-ECU.
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(1)Erase the diagnosis code.
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(2)Turn the ignition switch from "LOCK" (OFF) position to "ON" position.
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(3)On completion, check that the diagnosis code is set again.
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Q.
Is the diagnosis code set?
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Return to Step 1.
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The procedure is complete.
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