AU
B1642

DTC B1642 indicates a short to power (+B) in the driver seat belt pretensioner ignition circuit — Atto 8

Safety System

DTC B1642 indicates a short to power (+B) in the driver seat belt pretensioner ignition circuit.

The pretensioner is a key actuator in the SRS safety system containing a pyrotechnic igniter and a gas generator.

Normal static resistance is approximately 1.5-3.0Ω.

The SRS ECU sets this fault code upon detecting abnormal continuity between the pretensioner circuit and the vehicle power supply (12V constant power or IGN power).

This fault presents two risks: first, it can cause the pretensioner to deploy and tighten unexpectedly while driving, startling or injuring the driver; second, during an actual collision, the power supply clamps the circuit, preventing the pretensioner from receiving sufficient ignition current and neutralizing its protective function.

Additionally, this fault forces the SRS system into fail-safe mode, potentially disabling the entire airbag system.

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Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Worn wiring harness under the driver's seat: Forward and backward seat movement causes the pretensioner wiring harness to rub against the seat track or frame. This prolonged friction damages the insulation, causing the internal copper wire to contact and short-circuit to a power wire (such as the seat heater or seat power wire).
  • 2Floor wiring harness interference: A detached vehicle floor wiring harness retaining clip causes the pretensioner wiring harness and the main wiring harness power wire to chafe and short-circuit due to vehicle vibration.
  • 3Internal connector short circuit: Water ingress, corrosion, or bent pins in the pretensioner connector (usually yellow) under the driver's seat or lower B-pillar bridge the power and signal pins.
  • 4Clock spring internal fault: If the pretensioner circuit routes through the clock spring under the steering wheel, an internal short circuit between the flat cable layers may cause a short to power (on some vehicle configurations).
  • 5Improper modification or repair: When installing seat ventilation/heating or tapping power for a dash cam, improperly insulating the wiring harness or mistakenly connecting the power wire to the pretensioner circuit.
What To Do
  • 1
    Safety preparation: Switch off the vehicle, disconnect the negative battery cable, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Visual inspection: Move the driver seat to the fully forward and fully rearward positions. Inspect the pretensioner wiring harness (yellow corrugated conduit) under the seat, inside the B-pillar trim panel, and under the floor carpet for wear, cuts, or burn marks. Check the connector for looseness or water ingress.
  • 3
    Circuit isolation measurement: Disconnect the SRS ECU connector (located under the center console or bulkhead). Use a multimeter to measure continuity from the driver pretensioner circuit (typically pins B05-12 and B05-27; refer to the model wiring diagram) to body ground and to power (B+). Normal condition: Open circuit (infinite resistance).
  • 4
    Pretensioner unit test: Disconnect the pretensioner connector under the seat and measure the resistance between the two terminals of the pretensioner plug. The resistance must be 1.5-3.0Ω (room temperature). If the resistance is 0Ω or infinite, the pretensioner has an internal short or open circuit. Replace the pretensioner.
  • 5
    Sectional troubleshooting: If the circuit shorts to power, use the sectional isolation method (disconnect the intermediate connector) to determine if the short is in the SRS ECU-to-seat section or the seat-to-pretensioner section. Focus inspection on the wiring harness protection at the sharp edges of the seat frame.
  • 6
    Repair and verification: Repair the damaged wiring harness (use solder and heat-shrink tubing; simple wrapping is prohibited), replace damaged connectors, and securely fasten and correctly route the wiring harness. Reconnect the battery, clear the fault code, and perform an SRS system self-check to confirm B1642 does not return. Perform a simulated crash test (use the diagnostic tool to execute a crash output test and verify the pretensioner circuit functions normally).
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro - Seat rail chafed wiring harness causing short circuit

✓
The vehicle came in with the SRS warning light on steady. Read DTC B1642. Removed the driver's seat and found the yellow pretensioner harness beneath had been chafing against the metal edge of the seat rail over time. The insulation wore through, and the internal copper wires contacted the 12V power line to the seat heating module, creating a short to power. The technician repaired the harness by replacing a 20 cm section, repositioned the harness mounting points, and added corrugated tubing to prevent contact with metal edges during full seat movement. A three-month post-repair follow-up confirmed no recurrence.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song MAX - Connector shorted due to water ingress after wading

✓
The owner reported the SRS warning light illuminated after heavy rain. The scan tool showed code B1642: Driver Seat Belt Pretensioner Short to Power. Inspection found the seal on the pretensioner connector (yellow plug) beneath the driver's seat had aged, letting water in during wading that caused electrolytic corrosion between the pins inside the connector, shorting the power pin to the pretensioner drive pin. Repair procedure: thoroughly cleaned the corrosion from inside the connector, dried it with electronic contact cleaner, applied conductive anti-corrosion compound, and fitted a new waterproof seal. The corrosion was minor and the pretensioner resistance read normal, so replacement was not needed. Reassembled and cleared the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Tang DM – Wiring harness misconnection after accident repair

✓
After a front-end collision repair, the vehicle set B1642 immediately on startup. Reviewing the repair history, we found the driver seat assembly had been replaced during the repair. When connecting the wiring under the seat, the technician mistakenly swapped the pretensioner connector (yellow 2-pin) with the seat ventilation system power connector (black 2-pin, similar shape), sending 12V directly to the pretensioner. After correcting the connections, the fault code cleared. We used a dedicated diagnostic tool to code the SRS ECU and perform system calibration, restoring the pretensioner circuit monitoring voltage to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Yuan EV - Aftermarket seat heating modification caused wiring damage

✓
The SRS light illuminated one week after the owner fitted an aftermarket seat heating pad. Inspection found the 12V power wire for the heating pad (tapped from the cigarette lighter) crossed the pretensioner harness inside the seat foam. Inadequate securing caused the wires to chafe while driving, and the positive wire wore through and shorted to the pretensioner circuit. Repair: Removed the non-compliant modification wiring. Repaired the damaged original pretensioner harness (sealed with double-layer heat-shrink tubing). Advised the owner to use factory seat heating or a professional-grade modification with an independent fuse and relay, routing wiring away from SRS circuits.
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]