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DTC U0121: ASC CAN time-out
 
 
caution
  • If there is any problem in the CAN bus lines, an incorrect DTC may be stored. Prior to this diagnosis, always diagnose the CAN bus lines.
  • Before replacing the ECU, ensure that the communication circuit is normal.
 
 
OPERATION
AWD-ECU receives the data (such as ABS sensor signal, ABS operation signal) via the CAN bus lines. If it cannot receive such data, it interrupts the system.
 
 
DTC SET CONDITIONS
If the following conditions are met when the system is activated, AWD-ECU gradually switches the control from AWD to FWD, flashes the AWD/LOCK indicators alternately, and stores DTC U0121.
  • IG1 voltage: 10 V or more
  • ABS signal (such as wheel speed signal) cannot be received.
 
 
TROUBLESHOOTING HINTS
Current trouble
  • Wiring harness or connector failure between ASC-ECU and the stoplight switch
  • Malfunction of the ABS sensor
  • Malfunction of the stoplight switch
  • ASC-ECU malfunction
  • AWD-ECU malfunction
Past trouble
  • Refer to "How to treat past trouble" (GROUP 00 - How to Use Troubleshooting/How to Treat Past Trouble ) for proceeding the diagnostics.
 
 
DIAGNOSIS
STEP 1. Scan tool MB991958 CAN bus diagnostics
Use scan tool MB991958 to diagnose the CAN bus lines.
Is the check result normal?
Go to Step 2.
Repair the CAN bus line (Refer to GROUP 54C - Troubleshooting/CAN Bus Diagnostics Table ). Then go to Step 2.
 
STEP 2. Scan tool MB991958 data list
Check the following data list (Refer to ).
  • Item 07: CAN data (ABS/A.S.C.)
Is the check result normal?
Go to Step 6.
Go to Step 3.
 
STEP 3. Scan tool MB991958 DTC of other systems
Check if a DTC, which relates to the CAN communication-linked systems below, is stored.
  • Engine
    U0121: ASC time-out DTC
Is the DTC stored?
Go to Step 4.
Go to Step 5.
 
STEP 4. Check whether the DTC is stored again.
(1) Erase the DTC.
(2) Turn the ignition switch from the "LOCK" (OFF) position to the "ON" position.
(3) Check if the DTC is stored.
Is DTC U0121 stored?
Troubleshoot the ASC system (Refer to GROUP 35C - Troubleshooting/DTC Chart ). Then go to Step 5.
Go to Step 6.
 
STEP 5. Check whether the DTC is stored again.
(1) Erase the DTC.
(2) Turn the ignition switch from the "LOCK" (OFF) position to the "ON" position.
(3) Check if the DTC is stored.
Is DTC U0121 stored?
Replace AWD-ECU (Refer to ). Then go to Step 6 .
The procedure is complete.
 
STEP 6. Check whether the DTC is stored again.
Is DTC U0121 stored?
Diagnose again from Step 1.
The procedure is complete.