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DTC U0121: ASC-ECU Time-out

 
 
caution
  • If the ECM sets DTC U0121, make sure to diagnose the CAN bus line.
  • Before replacing the ECU, make sure that the communication circuit is operating normally.
 
 
DTC SET CONDITIONS
Check Conditions
  • After 2 seconds have passed since the ignition switch was turned to "ON" position.
  • Battery positive voltage is between 10 and 16.5 volts.
Judgment Criterion
  • Unable to receive ASC-ECU signals through the CAN bus line for 4 seconds.
 
 
COMMENT
Current Trouble
  • Some of the possible causes are a harness or connector damage between the ECM and the ASC-ECU on the CAN bus line, a failure in the ASC-ECU power supply system, a failure in the ASC-ECU, or a failure in the ECM.
Past Trouble
  • Proceed to troubleshoot based on a harness or connector damage on the CAN bus line between the ECM and ASC-ECU, and a failure in the ASC-ECU power supply system. Refer to "How to cope with past trouble" (Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Treat Past Trouble ).
noteIf a malfunction occurred in the past, a failure cannot be discovered through the scan tool (M.U.T.-IIISE) CAN bus diagnosis even if there might be a problem with the CAN bus. In this case, refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions . Furthermore, it is possible to narrow down the areas of the possible failures from the DTCs that are set in the ECUs, which are communicating on the CAN bus [Refer to GROUP 54C, Explanation about the Scan Tool (M.U.T.-IIISE) CAN Bus Diagnostics ].
 
 
FAIL-SAFE AND BACKUP FUNCTION
  • None.
 
 
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
  • CAN line harness damage or connector damage.
  • ASC-ECU failed.
  • ECM failed.

 
 
DIAGNOSIS
STEP 1. Using scan tool (M.U.T.-IIISE), diagnose CAN bus line.
cautionTo prevent damage to scan tool (M.U.T.-IIISE), always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool (M.U.T.-IIISE).
(1) Connect scan tool (M.U.T.-IIISE). Refer to HOW TO CONNECT THE SCAN TOOL (M.U.T.-IIISE) .
(2) Turn the ignition switch to the "ON" position.
(3) Diagnose CAN bus line.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is the CAN bus line normal?
Go to Step 2.
Repair the CAN bus line. Refer to GROUP 54C, Diagnosis - Can Bus Diagnostics Table . Then go to Step 6.
 
STEP 2. Using scan tool (M.U.T.-IIISE), read the diagnostic trouble code (DTC).
(1) Turn the ignition switch to the "ON" position.
(2) Read the ASC-DTC.
(3) Turn the ignition switch to the "LOCK" (OFF) position.
Is the ASC-DTC set?
Refer to GROUP 35C, Diagnosis - Diagnostic Trouble Code Chart .
Go to Step 3.
 
STEP 3. Using scan tool (M.U.T.-IIISE), read the diagnostic trouble code (DTC).
(1) Turn the ignition switch to the "ON" position.
(2) Check that the other ECU performing CAN communication with ASC-ECU sets DTC No. U0121. Refer to GROUP 54C, CAN Communication-related DTC (U-code) Table .
(3) Turn the ignition switch to the "LOCK" (OFF) position.
Is DTC U0121 set?
Go to Step 4.
Go to Step 5.
 
STEP 4. Using scan tool (M.U.T.-IIISE), read the diagnostic trouble code (DTC).
(1) Turn the ignition switch to the "ON" position.
(2) After the DFI and MFI-DTC has been deleted, read the DFI and MFI-DTC again.
(3) Turn the ignition switch to the "LOCK" (OFF) position.
Is DTC U0121 set?
Replace the ASC-ECU. Then go to Step 6.
It can be assumed that this malfunction is intermittent of CAN bus line between ECM and ASC-ECU. Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions .
 
STEP 5. Using scan tool (M.U.T.-IIISE), read the diagnostic trouble code (DTC).
(1) Turn the ignition switch to the "ON" position.
(2) After the DFI and MFI-DTC has been deleted, read the DFI and MFI-DTC again.
(3) Turn the ignition switch to the "LOCK" (OFF) position.
Is DTC U0121 set?
Replace the ECM (Refer to Removal and Installation ). Then go to Step 6.
It can be assumed that this malfunction is intermittent of CAN bus line between ECM and ASC-ECU. Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions .
 
STEP 6. Using scan tool (M.U.T.-IIISE), read the diagnostic trouble code (DTC).
(1) Turn the ignition switch to the "ON" position.
(2) After the DFI and MFI-DTC has been deleted, read the DFI and MFI-DTC again.
(3) Turn the ignition switch to the "LOCK" (OFF) position.
Is DTC U0121 set?
Retry the troubleshooting.
The inspection is complete.