AU
B1771A

DTC B1771A indicates the SRS (Supplemental Restraint System) ECU detects the RH rear seatbelt pretensioner circuit resistance is below the calibrated threshold — Seal U

Safety System

DTC B1771A indicates the SRS (Supplemental Restraint System) ECU detects the RH rear seatbelt pretensioner circuit resistance is below the calibrated threshold. (The standard value is typically 2.0 ± 0.5 Ω; a measured value below 1.2 Ω or near 0 Ω triggers this code).

As a key passive safety system actuator, the pretensioner contains a pyrotechnic combustion chamber and a resistance wire.

Low resistance indicates a short circuit risk (harness chafing to ground, water ingress at the connector, or an internal short from a melted resistance wire).

To prevent accidental deployment or upon determining actuator failure, the SRS ECU illuminates the airbag warning light and may disable the RH rear side airbag and pretensioner deployment.

This disables active tensioning protection for that seat during a collision, increasing occupant injury risk.

The B1771A fault code on the Seal U indicates the SRS ECU has detected low resistance in the right rear seatbelt pretensioner circuit, suggesting a possible short circuit. Severity is medium because the airbag warning light may come on and pretensioner protection for that seat may be disabled. Common causes include damaged wiring under the right rear seat or B-pillar trim, and water ingress or corrosion in the pretensioner connector. 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
  • 1Worn wiring harness under the right rear seat or inside the B-pillar trim: Frequent forward and backward seat movement or passengers stepping on the harness damages the insulation, shorting the positive and negative wires and causing an abnormal drop in resistance.
  • 2Pretensioner connector water ingress and oxidation: Vehicle wading, a blocked sunroof drain tube, or liquid seepage during deep interior cleaning causes a short circuit between connector terminals or abnormal resistance.
  • 3Pretensioner unit internal fault: Aging, moisture ingress, or manufacturing defects cause an inter-turn short circuit in the pyrotechnic device internal resistance wire, resulting in permanently low resistance.
  • 4SRS ECU detection circuit fault: A damaged control unit internal sampling resistor or A/D conversion circuit generates a false low-resistance signal.
  • 5Improper modification or accident repair: The seat frame pinches the wiring harness, or the connector is not fully locked when retrofitting seat heating, replacing the carpet, or performing post-accident repairs.
What To Do
  • 1
    Safety preparation: Use a BYD VDS2000 or Launch X431 diagnostic tool to read the fault code. Confirm B1771A is a current fault, not a history fault, and record the freeze frame data. Turn the ignition switch off. Disconnect the battery negative terminal and wait at least 90 seconds for the SRS capacitor to fully discharge.
  • 2
    Locate the component: Remove the right rear B-pillar lower trim panel (or seat outer trim panel, depending on vehicle structure) and locate the seat belt pretensioner assembly, usually a black cylindrical pyrotechnic device with a yellow two-color connector.
  • 3
    Visual inspection: Check the pretensioner connector for looseness, the terminals for green oxidation, and the wiring harness for damage or crush marks. Focus on the wiring harness sleeve at the seat slide rail mounting points.
  • 4
    Resistance measurement: Disconnect the pretensioner connector. Use a multimeter on the 200 Ω range to measure the resistance between the pretensioner-side terminals. The standard value is 1.8-2.5 Ω. If the resistance is below 1.2 Ω, replace the pretensioner. Measure the insulation resistance from the harness side to ground and to power. The resistance must be greater than 1 MΩ; otherwise, repair the wiring harness.
  • 5
    Circuit continuity check: Disconnect the SRS ECU connector (usually under the center console or in the front compartment). Measure wiring harness continuity between the pretensioner connector and the ECU. Verify no short to ground or short between wires.
  • 6
    Component replacement: If the pretensioner is faulty, replace it with a genuine part. (Note: The pretensioner is an explosive component. Follow dangerous goods transport and storage regulations. During replacement, never use a multimeter resistance setting to measure the new part directly. Use a dedicated diagnostic tool for activation and testing.)
  • 7
    System reset: After repair, reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code. Perform the 'SRS System Self-Learning' or 'Configure New Sensor' function. Confirm the fault code does not recur and verify the airbag warning light turns off normally.
Real-World Cases
BYD DTC AI Analysis

Worn seat rails shorted the wiring harness

✓
A 2019 Qin PRO DM with 32,000 km had the airbag warning light on. Diagnostic scanner reported DTC B1771A. Found: wiring harness beneath right rear seat chafed against slide rail mounting bracket, insulation worn through causing positive-to-negative short, resistance measured at 0.8Ω. Repaired harness, installed abrasion-resistant corrugated tubing, rerouted clear of moving parts. Fault resolved.
BYD DTC AI Analysis

Connector corrosion after water ingress caused fault

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2018 BYD Qin Pro petrol. Owner reported the airbag warning light stayed on after driving through deep water. Inspection found water staining and corrosion inside the right rear pretensioner connector, with insulation resistance between terminals measuring only 200Ω. Water entered through the door sill trim during wading due to a degraded connector seal. Repair: Cleaned the connector terminals, applied electronic contact cleaner, and fitted a replacement waterproof connector. Fault resolved.
BYD DTC AI Analysis

Connector not locked after accident repair

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A Qin PRO DM had its right rear seatbelt assembly replaced after collision repair, but the airbag warning light came on after delivery. DTC B1771A. The pretensioner connector was inserted but the locking tab hadn't clicked into place. Driving vibration caused intermittent contact resistance at the terminals (intermittent low resistance). The fault cleared after reinserting the connector firmly and confirming the locking tab was fully seated.
BYD DTC AI Analysis

Pretensioner internal resistance wire short circuit

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2019 BYD Qin Pro, no accidents or water ingress. DTC B1771A appeared suddenly. Disconnected the harness and measured the pretensioner body resistance directly: 0.6Ω. Internal resistance wire short—likely a batch quality defect. Replaced the right rear seatbelt pretensioner assembly (requires replacement with the seatbelt; pretensioners not available separately). Configured the new part and cleared the fault.
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.