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B1645

DTC B1645 indicates the driver-side seat belt pretensioner squib circuit resistance measures 0 Ω or close to 0 Ω, confirming a short circuit — Atto 3

Safety System

DTC B1645 indicates the driver-side seat belt pretensioner squib circuit resistance measures 0 Ω or close to 0 Ω, confirming a short circuit.

The pretensioner is a critical SRS (Supplemental Restraint System) component.

During a collision, it fires the squib to rapidly tighten the seat belt and secure the occupant in the seat.

A 0 Ω resistance typically indicates an internal short circuit in the pretensioner squib, a wiring harness short to ground, bridged connector terminals, or a faulty internal driver circuit in the SRS control module.

This fault forces the SRS into fail-safe mode and continuously illuminates the instrument panel airbag warning light.

In a collision, the affected pretensioner will fail to deploy.

This condition may also disrupt normal airbag deployment logic, creating a severe safety hazard.

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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1Pretensioner squib internal short circuit: Moisture ingress, aging, or manufacturing defects cause an internal bridge wire short circuit in the seat belt retractor pretensioner.
  • 2Harness short to ground: Floor harness insulation chafes near the seat slide rail, shorting the pretensioner power or signal wire to body ground.
  • 3Connector fault: Water ingress, terminal corrosion, or short circuit between terminals at the yellow SRS connector under the seat (usually located at the B-pillar or seat base), or terminal bridging due to improper connection during repairs.
  • 4Internal SRS ECU fault: Airbag control module internal pretensioner drive circuit breakdown or sampling circuit fault causes a false resistance reading of 0.
  • 5Improper seat modification or repair: Tools pierce the wiring harness or crush the connector during seat cover replacement, seat heating retrofitting, or seat adjustment, causing a short circuit.
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal. Wait at least 90 seconds (or the time specified in the repair manual) to allow the SRS backup capacitor to fully discharge and prevent accidental airbag deployment. Wear an anti-static wrist strap and disable the vehicle high-voltage system (for new energy vehicles).
  • 2
    Preliminary inspection: Visually check the airbag warning light status on the instrument panel. Use the diagnostic tool to read and record the complete fault codes. Confirm B1645 is a current fault (not a history fault). Check the yellow SRS connectors under the driver's seat and at the B-pillar for looseness, water ingress, corrosion, or foreign matter.
  • 3
    Circuit continuity test: Disconnect the SRS ECU connector (located at the center console or front bulkhead) and locate the driver pretensioner pins (usually D+ and D-). Use a digital multimeter to measure the resistance from the harness-side terminals to ground; resistance must be greater than 1 MΩ. Measure the resistance between the two terminals. If the reading is 0 Ω, the harness is shorted. Inspect the wiring along the route for damage.
  • 4
    Pretensioner resistance measurement: Disconnect the pretensioner connector under the seat and directly measure the pretensioner body resistance. The normal value is 1.5-3.0 Ω (refer to the vehicle repair manual for specific values). If the measurement is 0 Ω or close to 0 Ω, the pretensioner has an internal short circuit. Replace the seat belt retractor assembly (the pretensioner is usually not serviced separately).
  • 5
    ECU-side verification: If the wiring harness and pretensioner are normal, inspect the SRS ECU connector terminals for deformation or backing out. Connect the pretensioner to the ECU using a dedicated wiring harness in place of the original harness. If the fault disappears, the original harness has a hidden short circuit. If the fault remains, the SRS ECU has an internal fault.
  • 6
    Repair and replacement: Repair damaged wiring harness sections (use heat shrink tubing or replace the wiring harness) and replace the damaged pretensioner or SRS ECU. Note: When replacing the pretensioner, install new retaining bolts (usually single-use) and tighten to the specified torque (usually 7-10 N·m).
  • 7
    System Reset and Test: Reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code. Perform an SRS system self-diagnosis to confirm B1645 does not return. Slide the seat forward and backward, adjust the backrest, and simulate vehicle vibration to verify the fault does not recur. Finally, perform a seat belt function test to confirm the mechanical components operate normally.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Tang DM seat wiring harness wear caused B1645 fault

✓
The vehicle came in with the airbag warning light on solid. Pulled DTC B1645 (driver pretensioner resistance 0 ohms). Found the wiring harness sheath damaged at the seat rail, with the black signal wire copper exposed and shorting to the seat frame. Repaired the damaged harness (re-wrapped and added protective sleeving), rerouted the harness to avoid interference with moving parts, and cleared the fault. Frequent electric seat adjustment on this model causes harness fatigue wear at the rails.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Yuan EV: Water ingress in seat belt pretensioner connector causing short circuit

✓
The customer reported the airbag warning light illuminated after wading. The diagnostic tool showed active DTC B1645. Removing the driver's seat revealed water stains and green corrosion inside the yellow SRS connector at the base. Resistance between the terminals measured 0.2 Ω. During wading, water seeped in through the carpet; the aged connector seal then allowed water entry. We cleaned the connector terminals using precision electrical cleaner and compressed air, replaced the waterproof seal, and applied sealant around the connector to improve waterproofing. The fault was resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Internal short circuit in BYD Song MAX pretensioner

✓
Replaced the front bumper after a minor collision; B1645 then set. Measured the resistance between the pretensioner connector terminals under the seat: 0 Ω. Disconnected the connector and measured the pretensioner body resistance: still 0 Ω. Determined the pretensioner internal bridge wire had fused, causing a short. Suspected cause: pretensioner energized during collision but did not fully detonate, or electrostatic damage during parts replacement. Replaced the driver-side seat belt retractor assembly (including pretensioner) and re-matched the SRS system. The warning light went out and system operation returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro seat modification pinched wiring harness, causing short circuit.

✓
The customer installed aftermarket seat ventilation and heating, after which the airbag warning light came on. Inspection found the wiring harness had not been secured properly during the modification. The seat frame pinched it, damaging the insulation and causing a direct short between the positive and negative wires. The pretensioner circuit measured 0 Ω, but the pretensioner unit itself was undamaged. Repaired the harness: cut out the damaged section, soldered the wires, and sealed the joints with heat-shrink tubing. Rerouted the harness and secured it with cable ties to the outside of the seat frame to prevent it being pinched again. Cleared the fault codes, and the system returned to normal.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.
Data confidence: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself. Sources: [1]