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.

The B1771 fault code on the Seal U indicates the SRS control module has detected low resistance in the right rear seat belt pretensioner circuit, suggesting a short circuit. Severity is medium because the airbag warning light may stay on and the pretensioner may not deploy correctly in a crash. Common causes include an internal pretensioner short or damaged wiring shorting to ground. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Right rear seat belt pretensioner internal short circuit: internal igniter bridge wire breaks and shorts to ground, or moisture in the pyrotechnic charge abnormally lowers resistance.
  • 2Harness worn and shorted to ground: The wiring harness below the B-pillar or floor harness chafes against the metal body frame. Damaged insulation causes a short circuit between the positive and negative terminals or a short to ground.
  • 3Connector water ingress and oxidation: Rear floor flooding, aging seals, or water seepage during car washes causes electrochemical corrosion of the pretensioner connector pins (usually located below the B-pillar or on the seat side), creating a low-resistance path.
  • 4SRS control module internal fault: Faulty ECU internal sampling resistor, faulty A/D conversion circuit, or abnormal software calibration data causes a false low resistance reading.
  • 5Improper modification or repair: During seat cover installation, seat heater retrofitting, or seat removal/installation, tools pierce the wiring harness or connectors fail to seat fully, causing a short circuit between terminals.
What To Do
  • 1
    Safety Preparation and Diagnostic Confirmation: Use BYD VDS2000 or Launch X431 to read all DTCs. Confirm B1771 is an active fault, not a historical fault. Record freeze frame data (vehicle status, mileage, voltage). Disconnect the battery negative terminal and wait at least 90 seconds to fully discharge the SRS energy storage capacitor.
  • 2
    Visual inspection and connector check: Remove the right rear lower B-pillar trim panel and sill trim panel. Inspect the pretensioner connector (typically yellow with a shorting bar) for looseness, water ingress, corrosion, or backed-out pins. Inspect the wiring harness outer sheath for abrasion, burn marks, or rodent damage.
  • 3
    Resistance measurement and component isolation: Disconnect the pretensioner connector. Use a digital multimeter (minimum 10MΩ input impedance) to measure the resistance between the two terminals on the pretensioner body. The standard value is 2.0±0.3Ω. If <1.0Ω, replace the pretensioner assembly. If normal, measure the resistance to ground for both terminals on the wiring harness side. The value must be >10MΩ; otherwise, inspect the wiring harness for wear points.
  • 4
    Wiring harness repair and insulation test: If the wiring harness is damaged, wrap two layers of high-temperature insulating tape (Tesa 51036 or equivalent) to repair it. Replace the harness section if necessary. After repair, measure harness continuity and insulation resistance to ground and power supply to verify no short circuit risk.
  • 5
    SRS module inspection: If the pretensioner and wiring harness test normal, inspect the SRS module connector (typically located under the center tunnel) for oxidized or backed-out pins. Use an oscilloscope to check the pretensioner drive circuit waveform. Update the SRS module software or replace the control unit if necessary.
  • 6
    Component replacement and system configuration: Use genuine parts when replacing a faulty pretensioner. Do not measure the resistance of new or old parts, as the multimeter current may trigger the igniter. After replacement, perform 'Airbag System Configuration Write' (enter the VIN and vehicle configuration code) and clear the DTC.
  • 7
    System self-check and verification: Connect the battery, turn the key to the ON position, and verify the airbag warning light on the instrument cluster goes out after 6 seconds. Use the diagnostic tool to perform the 'SRS System Active Test' (if supported) and confirm no communication faults or new DTCs. Perform a road test to verify the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

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

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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.

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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

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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
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U overview →
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.