INSPECTION PROCEDURE 2: Communication with Scan Tool MB991958 is not Possible with the SRS System Only.

CIRCUIT OPERATION
The SRS-ECU is powered from the ignition switch (IG₁ ).
The SRS-ECU power is supplied from two circuits. Even if one circuit is shut off, the air bag can inflate.
The SRS system diagnosis can be done by connecting scan tool MB991502 to the data link connector.
The SRS-ECU judges how severe a collision is by detecting signals from the left and right front impact sensors and the analog G-sensor. If the impact is over a predetermined level, the SRS-ECU outputs an ignition signal. At this time, if the safing G-sensor is on, the SRS air bag will inflate.
TECHNICAL DESCRIPTION (COMMENT)
If communication is not possible with the SRS only, the cause is probably an open circuit in the on-board diagnostic output circuit of the SRS or in the power circuit (including ground circuit).
TROUBLESHOOTING HINTS
Damaged wiring harnesses or connectors
Malfunction of the SRS-ECU