DTC 21: Driver's Air Bag Module (Squib) System Fault 1 (Short Circuit between Terminals of the Squib Circuit): Diagnosis

Required Special Tools:

• MB991958: Scan Tool (MUT-III Sub Assembly)
  • MB991824: Vehicle Communication Interface (V.C.I.)
  • MB991827: MUT-III USB Cable
  • MB991911: MUT-III Main Harness B (Vehicles without CAN communication system)
• MB991865: Dummy resister
• MB991866: Resister harness

1. STEP 1. Using scan tool MB991958, read the diagnostic trouble code. 

   Q: Is DTC 34 set? 

   • YES:  Go to Step 2.
   • NO:  Go to  Step 3.
     Fig 1: Connecting Special Tool MB991911 To Data Link Connector

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2. STEP 2. Check SRS-ECU connector C-12. 

   Q: Is the connector correctly engaged? 

   • YES:  Go to Step 3.
   • NO:  Engage the connector correctly. Then go to  Step 8.
     Fig 2: Identifying SRS-ECU Connector C-12

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3. STEP 3. Check SRS-ECU connector C-12, clock spring connector C-205 and driver's air bag module connector C-201 (Using scan tool MB991958, read the diagnostic trouble code). 
   1. Disconnect the negative battery terminal.
   2. Disconnect connectors C-12, C-205 and C-201, and then reconnect them.
   3. Connect the negative battery terminal.
   4. Erase the diagnostic trouble code memory, and check the diagnostic trouble code.

   Q: Is DTC 21 set? 

   • YES:  Go to Step 4.
   • NO:  The procedure is complete. It is assumed that DTC 21 set as connector C-12, C-205 or C-201 was engaged improperly.
     Fig 3: Identifying SRS-ECU Connector C-12

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     Fig 4: Identifying Connector C-201, C-205

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4. STEP 4. Check the driver's air bag module (Using scan tool MB991958, read the diagnostic trouble code). 
   1. Disconnect the negative battery terminal.
   2. By sliding the A section (in the figure) of air bag module connector C-201 in the arrow direction, disconnect the connector.
      Fig 5: Identifying Air Bag Module Connector C-201

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   3. Connect special tool MB991865 to special tool MB991866.
      CAUTION: Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened.
   4. Insert special tool MB991866 into clock spring side air bag module connector C-201 by backprobing.
   5. Connect the negative battery terminal.
   6. Erase the diagnostic trouble code memory, and check the diagnostic trouble code.

   Q: Is DTC 21 set? 

   • YES:  Go to Step 5.
   • NO:  Replace the driver's air bag module (Refer to AIR BAG MODULES AND CLOCK SPRING  .) Then go to  Step 8.
     Fig 6: Inserting Special Tool MB991866 Into Clock Spring Side Air Bag Module Connector C-201

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5. STEP 5. Check the clock spring. (Using scan tool MB991958, read the diagnostic trouble code). 
   1. Disconnect the negative battery terminal.
   2. Disconnect the clock spring connector C-205.
   3. Connect special tool MB991865 to special tool MB991866.
      CAUTION: Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened.
   4. Insert special tool MB991866 into clock spring harness side connector C-205 (terminal No.3 and 4) by backprobing.
   5. Connect the negative battery terminal.
   6. Erase the diagnostic trouble code memory, and check the diagnostic trouble code.

   Q: Is DTC 21 set? 

   • YES:  Go to Step 6.
   • NO:  Replace the clock spring (Refer to AIR BAG MODULES AND CLOCK SPRING  .) Then go to  Step 8.
     Fig 7: Inserting Special Tool MB991866 Into Clock Spring Harness Side Connector C-205

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6. STEP 6. Check the driver's air bag module circuit at the SRS-ECU connector C-12. 
   1. Disconnect SRS-ECU connector C-12.
      Fig 8: Identifying SRS-ECU Connector C-12

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      DANGER 

      To prevents the air bag from deploying unintentionally, disconnect the clock spring connector C-205 to short the squib circuit. 

   2. Disconnect the clock spring connector C-205.
      Fig 9: Identifying Clock Spring Connector C-205

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      CAUTION: Insert an insulator such as a cable tie to a depth of 4mm (0.16 inch) or more, otherwise the short spring will not be released.
   3. Insert a cable tie [3 mm (0.12 inch) wide, 0.5 mm (0.02 inch) thick] between terminals 11,12 and the short spring to release the short spring.
      Fig 10: Identifying Connector C-12

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      CAUTION: Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened.
   4. Check for continuity between C-12 harness connector terminals 11 and 12.

      It should be open circuit.

   Q: Does continuity exist? 

   • YES:  Go to Step 7.
   • NO:  Erase the diagnostic trouble code memory, and check the diagnostic trouble code. If DTC 21 sets, replace the SRS-ECU (Refer to SRS CONTROL UNIT (SRS-ECU)  .) Then go to  Step 8.
     Fig 11: Checking Continuity Between C-12 Harness Connector Terminals 11 And 12

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7. STEP 7. Check the harness for short circuit between SRS-ECU connector C-12 (terminal No. 11 and 12) and clock spring connector C-205 (terminal No.3 and 4.) 

   Q: Are harness wires between SRS-ECU connector C-12 (terminal No.11 and 12) and clock spring connector C-205 (terminal No.3 and 4) in good condition? 

   • YES:  Go to Step 8.
   • NO:  Repair the harness wires between SRS-ECU connector C-12 and clock spring connector C-205. Then go to Step 8.
     Fig 12: Identifying SRS-ECU Connector C-12

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     Fig 13: Identifying Harness Connector C-205

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8. STEP 8. Recheck for diagnostic trouble code. 

   Q: Is DTC 21 set? 

   • YES:  Return to  Step 1.
   • NO:  The procedure is complete (If no malfunctions are found in all steps, an intermittent malfunction is suspected. Refer to HOW TO COPE WITH INTERMITTENT MALFUNCTION .)