AU
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 8

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.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Pretensioner squib internal short circuit: Moisture ingress, aging, or manufacturing defects caused an internal bridge wire short circuit in the seat belt retractor pretensioner.
  • 2Harness short to ground: Floor harness chafes and wears through the insulation near the seat slide rail, causing the pretensioner power wire or signal wire to short 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 incorrect connection during repairs causing terminal bridging.
  • 4Internal SRS ECU fault: A shorted pretensioner driver circuit or sampling circuit fault in the airbag control module causes a false resistance reading of 0.
  • 5Improper seat modification or repair: Tools pierced the wiring harness or crushed the connector when replacing seat covers, retrofitting seat heaters, or adjusting the seat, causing a short circuit.
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (or the time specified in the workshop manual) to fully discharge the SRS backup capacitor 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 instrument panel airbag warning light status. Use a diagnostic tool to read and record the complete fault codes, and 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; the resistance must be greater than 1MΩ. Measure the resistance between the two terminals. If the reading is 0Ω, the harness has a short circuit. Inspect the wiring for damage.
  • 4
    Pretensioner resistance measurement: Disconnect the pretensioner connector under the seat and directly measure the resistance of the pretensioner body. The normal value is 1.5-3.0 Ω (refer to the specific model repair manual). If the reading is 0 Ω or near 0 Ω, the pretensioner body has an internal short circuit. Replace the seat belt retractor assembly (the pretensioner is usually not available separately).
  • 5
    ECU-side verification: If the wiring harness and pretensioner are normal, inspect the SRS ECU connector for deformed or backed-out terminals. Substitute the original harness with a dedicated test harness to connect the pretensioner to the ECU. If the fault clears, the original harness has a hidden short circuit. If the fault persists, the SRS ECU has an internal fault.
  • 6
    Repair and Replacement: Repair damaged wiring harness sections (use heat-shrink tubing or replace the harness), or replace the damaged pretensioner or SRS ECU. Note: When replacing the pretensioner, install new mounting 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 and confirm B1645 does not return. Slide the seat fore and aft and adjust the backrest to simulate vehicle vibration, then confirm the fault does not recur. Finally, perform a seat belt function test to confirm the mechanical parts are normal.
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 8 Fault Codes
B1108For 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.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.
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]