AU
B1645-00

This DTC indicates the SRS control module detects the driver-side seat belt pretensioner squib circuit resistance at or near 0 ohms, indicating a short circuit — Seal U

Safety System

This DTC indicates the SRS control module detects the driver-side seat belt pretensioner squib circuit resistance at or near 0 ohms, indicating a short circuit.

As a key actuator in the passive safety system, the pretensioner ignites the squib during a collision to generate gas and tighten the seat belt, eliminating slack between the occupant and the belt.

A 0-ohm resistance indicates an internal short circuit in the squib, a short to ground in the wiring harness, or a fault in the internal driver circuit of the SRS module.

This fault causes the SRS system to enter degraded mode.

The driver-side airbag may fail to deploy properly, and the airbag warning lamp illuminates.

On some BYD new energy models, similar DTCs may relate to the high-voltage system (such as the traction battery charging circuit).

However, the official repair manual explicitly assigns B1645-00 to the SRS restraint system.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Driver seat belt pretensioner squib internal short circuit (blown internal bridge wire or damp propellant causing continuity)
  • 2Pretensioner wiring harness chafed or pinched, causing a short to body ground (commonly near the seat rail or inside the B-pillar trim).
  • 3SRS control module internal igniter driver circuit fault (MOSFET breakdown or filter capacitor short circuit)
  • 4Water ingress and corrosion in the pretensioner connector causing a short circuit between terminals (common after vehicle wading or car washing)
  • 5Non-professional modifications causing wiring harness damage (e.g., accidentally damaging the SRS wiring harness when installing seat heating or ventilation systems)
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use VDS or a dedicated diagnostic tool to read all DTCs, check for accompanying fault codes (such as B1644-00 Driver Pretensioner Resistance Too High), and record the freeze frame data.
  • 3
    Visual inspection: Remove the driver's seat and lower B-pillar trim panel. Check the pretensioner wiring harness for wear, damaged insulation, or crush marks. Check the connector for water ingress or corrosion.
  • 4
    Resistance measurement: Disconnect the SRS module from the pretensioner. Use a digital multimeter to measure the resistance between the pretensioner connector terminals (normal value: 1.5-3.0 Ω). Measure the resistance from each terminal to ground (should be > 1 MΩ).
  • 5
    Wiring harness insulation test: If pretensioner resistance is normal, measure wiring harness continuity and insulation to ground from the pretensioner to the SRS module. Focus inspection on the harness sleeve in the moving area under the seat.
  • 6
    Component replacement: If diagnostics confirm an internal short circuit in the pretensioner, replace the driver seat belt assembly (the pretensioner is usually not supplied separately); if the wiring harness is faulty, repair or replace the wiring harness; if suspecting the SRS module, perform a substitution test.
  • 7
    System reset: After repair, connect all components, clear the fault codes, and perform an SRS system self-check (turn the ignition switch ON and observe if the warning light turns off). Calibrate the crash sensor if necessary.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV300 seat belt pretensioner connector shorted after water wading

✓
The vehicle waded through water up to seat height. Afterwards, the instrument cluster displayed a constant SRS warning light. Diagnosis revealed DTCs B1645-00 and B1655-00 (passenger side pretensioner short circuit). Removed and inspected the driver’s seat and found water inside the pretensioner connector beneath it; the terminals had oxidised and were conducting. Cleaned the connector and blew it dry. Measured the pretensioner resistance as normal (2.1Ω), but the wiring harness showed only 0.3Ω resistance to earth. Replaced the pretensioner-to-SRS module wiring harness, which rectified the fault. Recommend checking the vehicle’s sealing and avoiding wading through water again.
BYD DTC AI AnalysisFrom Chinese market (translated)

After accident repair, aftermarket seatbelt assembly caused zero resistance.

✓
Repaired the left front collision damage and replaced the driver seatbelt assembly, but the instrument cluster displayed an airbag fault. Read DTC B1645-00. Inspected the new component and found it was aftermarket; resistance across the pretensioner connector terminals measured 0.2Ω (internal short). Replaced with an OEM driver seatbelt assembly and measured 2.3Ω. Cleared the DTC and the system returned to normal. SRS components must be OEM because aftermarket parts may have quality issues.
BYD DTC AI AnalysisFrom Chinese market (translated)

New energy vehicle high-voltage charging resistor fault (system false positive or coding conflict)

✓
A BYD EV logged DTC B1645, but inspection found the seatbelt pretensioner system normal with no short circuits. Further diagnosis identified a faulty traction battery pre-charge resistor; abnormally low resistance caused a false fault code or confusion in the fault code definition. Replaced the battery pre-charge resistor and performed battery balancing to clear the fault. In some early BYD EVs, code B1645 may involve the BMS; to differentiate, check the specific model year and system configuration.
Original source ↗
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]