AU
B1775

DTC B1775 indicates the SRS (Supplemental Restraint System) detects a short to ground in the right rear seat belt pretensioner circuit — Seal U

Safety System

DTC B1775 indicates the SRS (Supplemental Restraint System) detects a short to ground in the right rear seat belt pretensioner circuit.

The seat belt pretensioner is a pyrotechnic actuator containing an explosive device and a resistance wire.

Normal resistance is approximately 2-3 Ω.

When the SRS control module detects circuit resistance below the threshold (typically <1 Ω) or an abnormal voltage drop to ground, it identifies a short to ground.

This safety-critical fault causes: 1) the SRS to enter degraded mode, disabling right rear side impact protection; 2) continuous illumination of the airbag warning light; 3) a risk of unintended pretensioner deployment in extreme cases.

The pretensioner wiring harness routes through the right rear door sill trim and C-pillar, exposing it to seat movement, liquid ingress, or mechanical damage.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Long-term chafing of the wiring harness under the right rear seat: During seat fore/aft adjustment or folding, the pretensioner wiring harness rubs against the seat frame or floor, causing insulation damage and a short to body ground.
  • 2C-pillar interior trim panel wiring harness clip failure: A loose or missing C-pillar trim panel retaining clip allows the wiring harness to contact the metal body while driving, causing an intermittent or continuous short to ground.
  • 3Pretensioner connector water ingress: An aging right rear door seal or open window allows rainwater to enter, forming an electrolyte at the pretensioner connector (usually located under the seat or at the base of the C-pillar) and causing a short circuit between pins or to ground.
  • 4Modification or repair damage: Retaining screws pierce the pretensioner wiring harness when installing seat covers, floor sound insulation materials, or rear entertainment systems, or the seat track crushes an improperly secured harness.
  • 5Pretensioner internal short circuit: Resistance wire insulation inside the pretensioner body fails due to high temperatures or aging, causing an internal short to the metal housing (less common, typically accompanied by abnormally low resistance).
What To Do
  • 1
    Safe power-down and wait: Disconnect the low-voltage battery negative terminal. Wait at least 90 seconds to fully discharge the SRS capacitor and prevent triggering the pretensioner during diagnosis.
  • 2
    Freeze frame data analysis: Use the VDS2000 diagnostic tool to read freeze frame data (vehicle speed, collision signal, voltage, etc.) recorded when the fault occurred to confirm if it is an intermittent fault.
  • 3
    Physical and visual inspection: Remove the right rear sill trim panel and the lower C-pillar trim panel. Inspect the pretensioner wiring harness (typically featuring yellow sleeving) for wear, cuts, or scorch marks. Focus on the area beneath the seat mounting points and the C-pillar transition area.
  • 4
    Pretensioner resistance measurement: Disconnect the pretensioner connector (Note: Use a short-circuit protection plug). Use a digital multimeter to measure the pretensioner body resistance. Normal value: 2.0-3.0 Ω. If <1 Ω, replace the pretensioner assembly.
  • 5
    Harness insulation check: With the pretensioner connector disconnected, measure the resistance from the harness side to ground; the resistance must be >1 MΩ. Measure continuity between both ends of the harness (SRS ECU end and pretensioner end) and confirm no short to ground.
  • 6
    Short circuit location: If the wiring harness has a short circuit, use the sectional isolation method (cut the wire at the midpoint or disconnect the intermediate connector) to locate the exact short circuit position. Repair the wiring harness (use solder and heat-shrink tubing; do not simply twist wires together).
  • 7
    Component replacement: If the pretensioner shorts internally, replace the right rear seat belt assembly (the pretensioner typically integrates with the seat belt retractor). After replacement, remark the wiring harness routing and secure the harness firmly.
  • 8
    System reset and verification: Reconnect all connectors and restore power. Use the diagnostic tool to clear the fault code. Perform the 'SRS system self-check' and 'crash sensor test' (software simulation, without triggering pyrotechnic devices). Verify B1775 does not return and the warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Worn right rear seat wiring harness caused intermittent B1775 fault on Song MAX

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Vehicle: 2020 Song MAX, 38,000 km. Symptoms: Intermittent airbag warning light, worse on rough roads. VDS logged historic fault B1775. Diagnosis: Removed right rear seat. Found seatbelt pretensioner harness chafed by seat slide rail bracket edge at full forward position; copper core exposed and contacting body. Repair: Cut damaged section (approx. 15 cm). Extended with high-temperature solder. Double-layer heat shrink insulation. Rerouted harness and added rubber sleeve. Repositioned cable clips to clear moving parts. No recurrence after 3 months.
BYD DTC AI Analysis

Tang DM right rear pretensioner connector corroded and short-circuited after water ingress

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Vehicle: 2021 Tang DM, wading depth approximately 40 cm. Symptoms: Airbag warning light remained on the day after wading; VDS reported active fault B1775. Diagnosis: Found muddy water inside the right rear C-pillar lower pretensioner connector. Measured 0.8 Ω between pin 3 (ground side) and pin 1 (signal line), and 0.3 Ω from pin 1 to ground. Repair: Cleaned the connector thoroughly with electronic cleaner; insulation measured >10 MΩ after drying. Applied conductive grease and wrapped with waterproof tape. Checked and cleared blocked door seal drain holes. Cleared fault codes; system returned to normal.
BYD DTC AI Analysis

Aftermarket heated seat installation on Yuan EV damaged pretensioner wiring harness

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Vehicle: 2019 Yuan EV, aftermarket rear seat heating retrofit. Symptoms: One week post-modification, B1775 code appeared and would not clear. Diagnosis: Mounting screws for the heating pad pierced the yellow sleeve of the right rear pretensioner harness, shorting both positive and negative wires to the screw (vehicle ground). Pretensioner resistance measured 0.5Ω (possible internal damage). Repair: Replaced right rear seatbelt assembly (includes pretensioner). Rerouted harness clear of seat mounting points, secured with dedicated cable clamps, and added an insulating sleeve to the pretensioner connector. Recommendation: Advised owner to carry out seat-related modifications at a 4S dealership.
BYD DTC AI Analysis

Qin Pro: Wiring harness clip at right rear C-pillar detached, causing intermittent short circuit.

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Vehicle: 2020 Qin petrol variant, 52,000 km. Symptoms: Airbag warning light illuminates after rain or washing, clears in dry weather. Diagnosis: Removed C-pillar trim panel. Found pretensioner harness retainer clip broken; harness drooped against C-pillar metal reinforcement plate. Long-term micro-vibration wore through insulation. Ground resistance measured 200 Ω wet, >10 MΩ dry. Repair: Replaced C-pillar harness retainer clip. Wrapped worn harness section with fleece tape. Applied 3M insulating tape to C-pillar metal edges. Maintained harness-to-body metal clearance >20 mm. Verified insulation with spray test.
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.