B17921A - B17921A Front Right Seat Belt Pretensioner 2 Resistance Zero

DTC B17921A Technical Description Document

### Fault Depth Definition

Fault code B17921A belongs to the Body Domain Control or Airbag Controller diagnostic monitoring code for safety system components. This code specifically points to the Front Right Seat Belt Pretensioner 2, indicating the system has detected an abnormal circuit state in this component and judges its resistance value to be 0. In automotive electronic architecture, the seat belt pretensioner function is responsible for real-time feedback of the motor's physical position and rotational speed when a collision occurs. When the control unit detects that the impedance characteristics of the signal loop do not conform to expected standards (i.e., exhibiting short circuit characteristics), it generates this fault code. This belongs to a high-priority abnormal state in the airbag system, meaning the system cannot confirm the electrical integrity of this pretensioner, potentially affecting post-collision safety constraint logic.

### Common Fault Symptoms

When the B17921A fault occurs, the vehicle system and driver end can perceive the following specific manifestations:

• The Airbag Warning Light (SRS Light) on the instrument panel lights up, indicating failure of safety system function.
• System logs show a clear fault description: "Front Right Seat Belt Pretensioner 2 Function Failure".
• The airbag control system marks the seat belt restraint capability at this location as unavailable, potentially causing an inability to execute pre-tension actions under specific collision conditions.

### Core Fault Cause Analysis

Based on diagnostic logic and electrical characteristics, the physical or logical factors leading to the generation of this fault code can be classified into the following three dimensions:

• Hardware Component (Sensor/Actuator): The coil or ignition drive circuit inside the Front Right Seat Belt Pretensioner 2 has broken down, causing internal resistance to directly enter a short circuit state.
• Wiring/Connector (Physical Connection): Insulation layer damage on the harness connected to the front right seat causes ground short circuit, or metal short connection and poor soldering occur between connector pins, resulting in physical connection faults.
• Controller (Logic Computation): The driver circuit inside the airbag controller responsible for detecting impedance makes a false judgment, erroneously identifying normal resistance as a short circuit signal.

### Technical Monitoring & Trigger Logic

The generation of this fault code is based on the real-time online monitoring mechanism of the airbag controller, with its determination logic as follows:

• Monitoring Target: Signal loop ground impedance characteristics (Resistance Value) of the seat belt pretensioner component.
• Numerical Threshold Range: The setting condition is strictly limited to Front Right Seat Belt Pretensioner 2 resistance value of $0$.
• Trigger Condition: During ignition switch on or system self-check cycle, when the airbag controller receives signal voltage or impedance feedback from Front Right Seat Belt Pretensioner 2 indicating short circuit (i.e., detecting resistance value of $0 \Omega$), it determines component electrical integrity is damaged, then generates fault code B17921A and stores it in control unit fault memory.