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

OPERATION

The SRS-ECU judges how severe a collision is by detecting signals from the front impact sensors and the front air bag analog G-sensor. If the impact is over a predetermined level, the SRS-ECU outputs an ignition signal. At this time, if the front air bag safing G-sensor is on, the SRS air bag will inflate.

The ignition signal is input to the air bag module via the clock spring to inflate the air bag.

DIAGNOSIS CODE SET CONDITIONS

This diagnosis code is set if there is abnormal resistance between the input terminals of the driver's side air bag module (squib). The most likely causes for this code to be set the followings:

Short circuit in driver’s air bag module (squib) or harness

Short circuit in the clock spring

However, if no diagnosis code resets, the SRS warning lamp will be switched off (diagnosis code will be retained).

PROBABLE CAUSES

Improper engaged connector or defective short spring*

Short circuit in the clock spring

Short circuit between the driver's air bag module (squib) circuit terminals

Damaged connector(s)

Malfunction of the SRS-ECU

NOTE: *: The squib circuit connectors integrate a "short" spring (which prevents the air bag from deploying unintentionally due to static electricity by shorting the positive wire to the earth wire in the squib circuit when the connectors are disconnected). Therefore, if connector C-115, C-307 or C-306 is damaged or improperly engaged, the short spring may not be released when the connector is connected.

DIAGNOSIS PROCEDURE

STEP 1. MUT-II/III diagnosis code

CAUTION

To prevent damage to MUT-II/III, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting MUT-II/III.

Turn the ignition switch to the "ON" position.

Check if the diagnosis code is set.

Turn the ignition switch to the "LOOK" (OFF) position.

Q: Is diagnosis code 34 set?

YES: Go to Step 2

.
NO: Go to Step 3

STEP 2. Connector check: SRS-ECU connector C-115

Q: Is the connector correctly engaged?

YES: Go to Step 3

.
NO: Engage the connector correctly. Then go to Step 8

STEP 3. Connector check: SRS-ECU connector C-115, clock spring connector C-307 and driver’s air bag module connector C-306 (Check whether the diagnosis code is reset).

Disconnect the negative battery terminal.

Disconnect connectors C-115, C-307 and C-306, and then reconnect them.

Connect the negative battery terminal.

Erase the diagnosis code memory, and check the diagnosis code.

Q: Is diagnosis code 21 set?

YES: Go to Step 4

.
NO: The procedure is complete. It is assumed that diagnosis code 21 set as connector C-115, C-307 or C-306 was engaged improperly.

STEP 4. Check the driver's air bag module (Check whether the diagnosis code is reset).

Disconnect the negative battery terminal.

By sliding the A section (in the figure) of air bag module connector C-306 in the arrow direction, disconnect the connector.

Connect special tool dummy resistor (MB991865) to special tool resistor harness (MB991866).

CAUTION

Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened.

Insert special tool MB991866 into clock spring side air bag module connector C-306 by backprobing.

Connect the negative battery terminal.

Erase the diagnosis code memory, and check the diagnosis code.

Q: Is diagnosis code 21 set?

YES: Go to Step 5

.
NO: Replace the driver's air bag module (Refer to

). Then go to Step 8

STEP 5. Check the clock spring. (Check whether the diagnosis code is reset).

Disconnect the negative battery terminal.

Disconnect the clock spring connector C-307.

Connect special tool dummy resistor (MB991865) to special tool resistor harness (MB991866).

CAUTION

Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened.

Insert special tool MB991866 into clock spring harness side connector C-307 (terminal No.3 and 4) by backprobing.

Connect the negative battery terminal.

Erase the diagnosis code memory, and check the diagnosis code.

Q: Is diagnosis code 21 set?

YES: Go to Step 6

.
NO: Replace the clock spring (Refer to

). Then go to Step 8

STEP 6. Check the driver’s air bag module circuit at the SRS-ECU connector C-115.

Disconnect SRS-ECU connector C-115.

DANGER

To prevents the air bag from deploying unintentionally, disconnect the clock spring connector C-307 to short the squib circuit.

Disconnect the clock spring connector C-307.

CAUTION

Insert an insulator such as a cable tie to a depth of 4 mm or more, otherwise the short spring will not be released.

Insert a cable tie [3 mm wide, 0.5 mm thick] between terminals 11, 12 and the short bar to release the short spring.

CAUTION

Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened.

Check for continuity between C-115 harness side connector terminals 11 and 12.

OK: Open circuit

Q: Is the check result normal?

YES: Erase the diagnosis code memory, and check the diagnosis code. If diagnosis code 21 sets, replace the SRS-ECU (Refer to

). Then go to Step 8

.
NO: Go to Step 7

STEP 7. Check the harness for short circuit between SRS-ECU connector C-115 (terminal No.11 and 12) and clock spring connector C-307 (terminal No.3 and 4).

Q: Are harness wires between SRS-ECU connector C-115 (terminal No.11 and 12) and clock spring connector C-307 (terminal No.3 and 4) in good condition?

YES: Go to Step 8

.
NO: Repair the harness wires between SRS-ECU connector C-115 and clock spring connector C-307. Then go to Step 8

STEP 8. Check whether the diagnosis code is reset.

Q: Is diagnosis code 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 GROUP 00 - How to Use Troubleshooting/Inspection Service Points


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

OPERATION The SRS-ECU judges how severe a collision is by detecting signals from the front impact sensors and the front air bag analog G-sensor. If the impact is over a predetermined level, the SRS-ECU outputs an ignition signal. At this time, if the front air bag safing G-sensor is on, the SRS air bag will inflate. The ignition signal is input to the air bag module via the clock spring to inflate the air bag.
DIAGNOSIS CODE SET CONDITIONS This diagnosis code is set if there is abnormal resistance between the input terminals of the driver's side air bag module (squib). The most likely causes for this code to be set the followings: Short circuit in driver’s air bag module (squib) or harness Short circuit in the clock spring However, if no diagnosis code resets, the SRS warning lamp will be switched off (diagnosis code will be retained).
PROBABLE CAUSES Improper engaged connector or defective short spring* Short circuit in the clock spring Short circuit between the driver's air bag module (squib) circuit terminals Damaged connector(s) Malfunction of the SRS-ECU
NOTE: *: The squib circuit connectors integrate a "short" spring (which prevents the air bag from deploying unintentionally due to static electricity by shorting the positive wire to the earth wire in the squib circuit when the connectors are disconnected). Therefore, if connector C-115, C-307 or C-306 is damaged or improperly engaged, the short spring may not be released when the connector is connected.
	DIAGNOSIS PROCEDURE

	STEP 1. MUT-II/III diagnosis code CAUTION To prevent damage to MUT-II/III, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting MUT-II/III. Turn the ignition switch to the "ON" position. Check if the diagnosis code is set. Turn the ignition switch to the "LOOK" (OFF) position. Q: Is diagnosis code 34 set? YES: Go to Step 2 . NO: Go to Step 3.
	STEP 2. Connector check: SRS-ECU connector C-115 Q: Is the connector correctly engaged? YES: Go to Step 3 . NO: Engage the connector correctly. Then go to Step 8.
	STEP 3. Connector check: SRS-ECU connector C-115, clock spring connector C-307 and driver’s air bag module connector C-306 (Check whether the diagnosis code is reset). Disconnect the negative battery terminal. Disconnect connectors C-115, C-307 and C-306, and then reconnect them. Connect the negative battery terminal. Erase the diagnosis code memory, and check the diagnosis code. Q: Is diagnosis code 21 set? YES: Go to Step 4 . NO: The procedure is complete. It is assumed that diagnosis code 21 set as connector C-115, C-307 or C-306 was engaged improperly.
	STEP 4. Check the driver's air bag module (Check whether the diagnosis code is reset). Disconnect the negative battery terminal.
	By sliding the A section (in the figure) of air bag module connector C-306 in the arrow direction, disconnect the connector.
	Connect special tool dummy resistor (MB991865) to special tool resistor harness (MB991866). CAUTION Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened. Insert special tool MB991866 into clock spring side air bag module connector C-306 by backprobing. Connect the negative battery terminal. Erase the diagnosis code memory, and check the diagnosis code. Q: Is diagnosis code 21 set? YES: Go to Step 5. NO: Replace the driver's air bag module (Refer to ). Then go to Step 8.
	STEP 5. Check the clock spring. (Check whether the diagnosis code is reset). Disconnect the negative battery terminal.
	Disconnect the clock spring connector C-307. Connect special tool dummy resistor (MB991865) to special tool resistor harness (MB991866). CAUTION Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened. Insert special tool MB991866 into clock spring harness side connector C-307 (terminal No.3 and 4) by backprobing. Connect the negative battery terminal. Erase the diagnosis code memory, and check the diagnosis code. Q: Is diagnosis code 21 set? YES: Go to Step 6. NO: Replace the clock spring (Refer to ). Then go to Step 8.
	STEP 6. Check the driver’s air bag module circuit at the SRS-ECU connector C-115. Disconnect SRS-ECU connector C-115. DANGER To prevents the air bag from deploying unintentionally, disconnect the clock spring connector C-307 to short the squib circuit. Disconnect the clock spring connector C-307.
	CAUTION Insert an insulator such as a cable tie to a depth of 4 mm or more, otherwise the short spring will not be released. Insert a cable tie [3 mm wide, 0.5 mm thick] between terminals 11, 12 and the short bar to release the short spring.
	CAUTION Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened. Check for continuity between C-115 harness side connector terminals 11 and 12. OK: Open circuit Q: Is the check result normal? YES: Erase the diagnosis code memory, and check the diagnosis code. If diagnosis code 21 sets, replace the SRS-ECU (Refer to ). Then go to Step 8 . NO: Go to Step 7.
	STEP 7. Check the harness for short circuit between SRS-ECU connector C-115 (terminal No.11 and 12) and clock spring connector C-307 (terminal No.3 and 4). Q: Are harness wires between SRS-ECU connector C-115 (terminal No.11 and 12) and clock spring connector C-307 (terminal No.3 and 4) in good condition? YES: Go to Step 8 . NO: Repair the harness wires between SRS-ECU connector C-115 and clock spring connector C-307. Then go to Step 8.
	STEP 8. Check whether the diagnosis code is reset. Q: Is diagnosis code 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 GROUP 00 - How to Use Troubleshooting/Inspection Service Points .