AU
B1642-00

DTC B1642-00 indicates the driver-side seat belt pretensioner ignition circuit shorts to the vehicle power supply (B+) — Seal U

Safety System

DTC B1642-00 indicates the driver-side seat belt pretensioner ignition circuit shorts to the vehicle power supply (B+).

In the BYD SRS (Supplemental Restraint System) architecture, the pretensioner operates as a pyrotechnic actuator.

Normally, both terminals exhibit a high-resistance state (open circuit) to power and ground.

This fault indicates a wire in the pretensioner harness or connector shorts to constant power.

This condition can cause: 1) the SRS ECU to detect abnormal voltage, illuminate the airbag warning lamp, and disable the airbag system; 2) the pretensioner to deploy accidentally in extreme cases; 3) the pretensioner to fail during a collision, endangering occupant safety.

This fault involves the dedicated yellow SRS harness under the driver's seat, an area prone to mechanical wear from fore-aft seat movement.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Worn SRS wiring harness under the driver's seat rail: Frequent fore-and-aft seat adjustment causes the seat rail edge to cut the harness insulation, creating a short circuit with body power wires (such as the seat heating power wire or constant power memory wire).
  • 2Water ingress at the floor wiring harness connector: Water entering the driver's footwell from wading or liquid seepage during cleaning causes an electrolytic short circuit between the pretensioner connector pins (typically located under the seat or lower B-pillar), especially when foreign debris bridges the pins.
  • 3SRS ECU internal drive circuit fault: Airbag control module internal ignition drive transistor breakdown causes the pretensioner port to continuously output supply voltage.
  • 4Foreign object under the seat: Metal objects (such as coins or keys) or aftermarket wiring harnesses (such as dashcam hardwire kits or seat ventilation retrofit harnesses) crush the SRS wiring harness, causing insulation damage and a short to power.
  • 5Pretensioner body internal short circuit: The igniter inside the seat belt retractor pretensioner shorts to the housing (less common, usually accompanied by abnormal resistance).
What To Do
  • 1
    Safety preparation: Turn off the ignition, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Visual inspection: Remove the driver's seat (leave the wiring harness connected). Inspect the yellow SRS wiring harness sleeve under the seat rail for wear. Inspect the connector under the seat (usually marked DAB or Pretensioner) for water ingress, corrosion, or bent pins.
  • 3
    Circuit measurement: Disconnect the connector between the SRS ECU and the pretensioner. Use a multimeter to measure the resistance from the pretensioner-side wiring harness to body ground and battery positive (B+). Normal resistance is infinite (OL). A low resistance reading indicates the wiring harness is shorted to power.
  • 4
    Harness isolation test: Strip the harness sleeve section by section along the pretensioner harness routing (from below the B-pillar, across the floor to the seat) and inspect for damage. Focus on intersections with the seat heating harness and memory power supply harness.
  • 5
    Component replacement: If the wiring harness has no damage, measure the pretensioner resistance (normal: approx. 2-3 Ω). If the resistance is abnormal, replace the pretensioner assembly. If both the wiring harness and pretensioner are normal, replace the SRS ECU.
  • 6
    System reset: After repair, reconnect all connectors and the battery. Use VDS2000/3000 or Launch X431 to perform SRS system configuration and coding. Clear fault codes and perform static/dynamic testing.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Worn seat rails caused SRS wiring harness short circuit on Qin EV300

✓
A 2017 Qin EV300 with 80,000 km had the airbag warning light on. The system stored fault code B1642-00. Inspection found the wiring harness sheath behind the driver's seat rail had split; the power wire in the yellow SRS harness and the seat heater feed (red) had chafed through their insulation from prolonged friction and adhered together. The repair: mended the damaged harness, rewrapped it with high-temperature corrugated tubing, and repositioned the harness clips to clear the seat rail. This cleared the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin 100 pretensioner connector corroded and shorted after wading through water

✓
One week after the water wading repair, a B1642-00 fault appeared. Inspection found sediment residue inside the SRS connector under the driver's seat. Electrolyte had formed between the pins, shorting the power and signal pins. During the earlier water wading incident, water had risen above the floor, but the connector was not thoroughly cleaned during the repair. Resolution: Thoroughly cleaned the connector with electronic cleaner, blow-dried it, applied conductive protective agent, and reinstalled once insulation resistance values returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

Aftermarket seat ventilation modification caused pretensioner circuit interference

✓
A third-party modification shop installed a seat ventilation system. When tapping power, the installer mistakenly used the pretensioner harness as an earth or permanent live, causing DTC B1642-00 and multiple SRS fault codes. Inspection found aftermarket wiring beneath the seat; the ventilation system positive wire was connected directly in parallel with the pretensioner harness. Resolution: Removed the unauthorised wiring, restored the factory harness connections, replaced the damaged SRS ECU fuse, and reconfigured the seat occupancy recognition system using the factory diagnostic tool.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV450 SRS ECU internal driver fault

✓
No accident history. DTC B1642-00 appeared intermittently, returning days after clearing. Checked wiring harness insulation – good. Measured pretensioner resistance at 2.1Ω (normal). Suspected a fault in the SRS ECU internal ignition driver circuit. Oscilloscope monitoring detected 12V output at the pretensioner port while the ECU was in sleep mode. Replaced the SRS ECU (performed VIN matching and configuration coding). Fault completely resolved.
Other Seal U 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]