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DTC U0121: ASC CAN timeout
 
 
caution
  • If DTC U0121 is set, diagnose the CAN bus lines.
  • When replacing the ECU, always check that the SRS circuit is normal.
 
 
FUNCTION
If signal from the ASC-ECU cannot be received, the SRS-ECU sets DTC U0121.
 
 
TROUBLE JUDGMENT
Because of the CAN-C bus circuit malfunction, if SRS-ECU becomes unable to perform the normal data transmission, SRS-ECU determines that an abnormality is present.
 
 
PROBABLE CAUSES
  • Malfunction of CAN bus line
  • Malfunction of ASC-ECU
  • Malfunction of SRS-ECU
 
 
DIAGNOSIS
STEP 1. Using scan tool (M.U.T.-IIISE), diagnose the CAN bus line.
(1)
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).
Connect scan tool (M.U.T.-IIISE). Refer to "How to connect the scan tool ."
(2) Turn the ignition switch to the "ON" position.
(3) Diagnose the CAN bus line.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is the CAN bus line found to be normal?
Go to Step 2.
Repair the CAN bus line (Refer to GROUP 54C, Diagnosis ).
 
STEP 2. Using scan tool (M.U.T.-IIISE), check whether the other system DTC is set.
Check the ASC-ECU DTC.
Is the DTC set?
Diagnose the ASC-ECU (Refer to GROUP 35C - Diagnosis ).
Go to Step 3.
 
STEP 3. Using scan tool (M.U.T.-IIISE), check whether the DTC is set again.
Check the DTC.
(1) Erase the diagnostic trouble code.
(2) Turn the ignition switch from "LOCK" (OFF) position to "ON" position.
(3) On completion, check that the DTC is set again.
Is the DTC set?
Replace the ASC-ECU (Refer to GROUP 35C - Hydraulic unit ). After replacing the ECU, carry out the CAN bus diagnostics. Then go to Step 4.
A poor connection, open circuit or other intermittent malfunction is present in the CAN bus lines between the ASC-ECU and the SRS-ECU (Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).
 
STEP 4. Using scan tool (M.U.T.-IIISE), check whether the DTC is set again.
Check the DTC.
(1) Erase the diagnostic trouble code.
(2) Turn the ignition switch from "LOCK" (OFF) position to "ON" position.
(3) On completion, check that the DTC is not set again.
Is the DTC set?
Replace the SRS-ECU (Refer to ). After replacing the ECU, carry out the CAN bus diagnostics.
A poor connection, open circuit or other intermittent malfunction is present in the CAN bus lines between the ASC-ECU and the SRS-ECU (Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).