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AU
B1771

DTC B1771 indicates the SRS (Supplemental Restraint System) control module detects the right rear seat belt pretensioner resistance is below the system-calibrated lower threshold (typically <1 — Seal U

Safety System

DTC B1771 indicates the SRS (Supplemental Restraint System) control module detects the right rear seat belt pretensioner resistance is below the system-calibrated lower threshold (typically <1.0Ω or near a short-circuit value).

The pretensioner is a pyrotechnic actuator containing an electric squib with a normal resistance of approximately 2.0±0.3Ω.

Low resistance indicates a pretensioner internal short circuit, a wiring harness short to ground, connector water ingress causing electrochemical corrosion, or an SRS module internal sampling circuit fault.

This fault triggers the airbag system fail-safe strategy.

The affected seat pretensioner may fail to deploy in a collision, the AIRBAG warning lamp remains illuminated, and some models trigger a system buzzer alarm.

4
Cases Logged
Real-World Cases
BYD DTC AI Analysis

Repaired short circuit in right rear seat belt pretensioner on water-damaged vehicle

✓
Vehicle: 2020 BYD Song Pro DM. Fault: After heavy rain, the airbag warning light stayed on constantly. DTC B1771 (current). Found the right rear carpet wet. Removed the B-pillar trim panel and found obvious water inside the pretensioner connector; the pins were oxidized and blackened. Cleaned the connector with alcohol and dried it. The pretensioner body measured normal resistance (1.9Ω), but resistance between the connector terminals was only 0.3Ω. Replaced the wiring harness connector and waterproofed it. Fault cleared. Recommend checking the sunroof drain hoses and door seals.
BYD DTC AI Analysis

Seat modification caused wiring harness wear.

✓
Model: 2019 Yuan EV535. Airbag warning light illuminated after fitting leather seat covers. Diagnosis revealed intermittent DTC B1771. Removed the right rear seat and found the installer had accidentally driven securing pins through the pretensioner wiring harness when fitting the seat covers, shorting the positive and negative wire cores. Repaired the damaged harness (2 wires, 0.5mm²) and rerouted it to avoid the seat rail travel path. Fault resolved. Note: Disconnect the battery and mark pretensioner harness positions when removing or fitting seats.
BYD DTC AI Analysis

Replaced pretensioner due to internal short circuit

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Model: 2021 Tang DM-i. Symptom: No collision history, but airbag warning lamp suddenly illuminated while driving; DTC B1771 present. Disconnected pretensioner connector and measured unit resistance at only 0.6Ω—far below specification. Diagnosis: Internal pretensioner igniter short. Repair: Replaced right rear seatbelt assembly (pretensioner integrated with retractor), performed system configuration write. Fault resolved. Findings: Disassembly of old part revealed internal propellant had absorbed moisture and caked, causing bridge wire to short against housing.
BYD DTC AI Analysis

Software fix for SRS module false positive

✓
Vehicle: 2019 Qin Pro EV. Fault: DTC B1771 occurs intermittently, with increased frequency during humid weather. Diagnosis: Measured pretensioner and wiring harness resistance within specification; inspected connectors with no faults found. Reviewed technical bulletins revealing an SRS software calibration defect in this production batch causing false low-resistance readings during voltage fluctuations. Repair: Upgraded SRS control module software to V2.3.5 or above. Result: Fault has not reoccurred. Software updates are critical for diagnosis.
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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.