AU
B1773-00

DTC B1773-00 indicates the airbag control unit (SRS ECU) failed to detect the electrical signal or communication response from the right rear seat belt pretensioner during the system self-check — Seal U

Safety System

DTC B1773-00 indicates the airbag control unit (SRS ECU) failed to detect the electrical signal or communication response from the right rear seat belt pretensioner during the system self-check.

This passive safety system hardwire/communication fault indicates an abnormality in the squib circuit between the SRS module and the right rear seat belt pretensioner.

Causes include an open circuit, a short to ground or power, or the pretensioner internal igniter resistance falling outside the standard range (typically 1.5-3.5Ω).

This fault prevents the pretensioner from deploying and tightening the seat belt during a collision, significantly reducing occupant protection.

The SRS system also enters fail-safe mode, which may disable other airbags and illuminate the airbag warning light continuously.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Right rear seat belt pretensioner connector (usually a yellow waterproof plug) is loose, disconnected, or has poor contact. Common causes include frequent forward and backward seat movement, incomplete insertion after removal, or a broken locking tab.
  • 2Open circuit or abnormal resistance in the internal pretensioner igniter (aging, moisture ingress, previous deployment, or manufacturing defect), preventing the SRS module from detecting the standard resistance circuit.
  • 3Physical damage to the wiring harness causing an open or short circuit, especially wiring pinched by the under-seat rails, wear points at bends inside the B-pillar trim panel, or fatigue fractures at the door hinge wiring pass-through.
  • 4High resistance, intermittent open circuit, or connector pin oxidation in the communication circuit between the SRS control module and the pretensioner (especially in high-humidity areas or after wading).
  • 5The pretensioner deployed during an accident and remains unreplaced, or an installed non-genuine part caused a resistance mismatch, triggering the system to identify it as 'not present'.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds for the SRS capacitor to discharge fully, preventing accidental airbag deployment. Wear an anti-static wrist strap and prepare the BYD dedicated diagnostic tool (VDS2000/ED400).
  • 2
    Fault confirmation: Connect the diagnostic tool and read the DTCs. Confirm B1773-00 is a current fault (Active), not a historical fault. Record the freeze frame data. Check for accompanying communication fault codes (such as B1772, B1774, etc.).
  • 3
    Connector check: Remove the right rear seat and lower B-pillar trim panel. Verify the yellow pretensioner connector connects securely. Confirm the locking tab (CPA) locks fully into position. Inspect inside the connector for moisture, oxidation, or backed-out pins.
  • 4
    Resistance measurement: Disconnect the pretensioner connector. Measure the resistance between the pretensioner terminals using a multimeter. The standard value is 1.5-3.5Ω (refer to the specific model workshop manual). Infinite resistance indicates an internal open circuit and 0Ω indicates an internal short circuit; replace the assembly in either case.
  • 5
    Wiring harness continuity test: Measure wiring harness resistance between the SRS module and the pretensioner; resistance must be less than 1 Ω. Measure wiring harness insulation resistance to ground and power; resistance must be infinite. Inspect seat rail pinch points and body pass-through holes.
  • 6
    Component replacement and verification: If the pretensioner is faulty, replace the right rear seat belt assembly (includes the pretensioner; do not replace the igniter separately). If the wiring harness is faulty, repair the harness. After reassembly, clear the fault code and perform an SRS system self-check (turn the ignition switch ON and observe if the warning lamp turns off after 6 seconds). If necessary, execute the diagnostic tool's 'Crash Output Test' function to verify circuit integrity.
Real-World Cases
BYD DTC AI Analysis

Intermittent fault caused by pretensioner connector not locked after seat removal

✓
2018 BYD Qin EV450, airbag warning light illuminates intermittently. VDS read DTC B1773-00 (intermittent). Owner reported removing the seats for deep cleaning at a detailing shop one week prior. Inspection found the yellow connector for the right rear seatbelt pretensioner inserted but the CPA (Connector Position Assurance) lock tab not fully seated; vehicle vibration caused poor contact. Reseated the connector and confirmed the CPA lock engaged. Cleared the DTC and monitored for one week with no recurrence.
BYD DTC AI Analysis

Seat rails crushed wiring harness causing intermittent open circuit

✓
2017 BYD Qin 80. Airbag warning light illuminated randomly while driving. Scanner showed B1773-00 as a current/history alternating fault. Pretensioner resistance measured normal at rest, but the reading fluctuated when manually sliding the seat fore and aft. Disassembly revealed the wiring harness insulation had been cut by the seat rail edge at the mounting point, with copper conductors partially fractured leaving only a few strands intact. Resoldered the harness and fitted abrasion-resistant corrugated tubing, repositioned the retaining clip to prevent contact with moving components. Fault permanently resolved.
BYD DTC AI Analysis

Water ingress caused internal short circuit in pretensioner, triggering fault code

✓
A Qin 100 drove through floodwater after heavy rain (water depth exceeded the door sills). The airbag warning light stayed on. Pulled DTC B1773-00. Measured pretensioner resistance at 0.2Ω—well below spec, confirming an internal short. Disassembly revealed the pretensioner housing seal had degraded, allowing water to enter; the internal igniter got wet and the insulation between electrodes failed. Replaced the right rear seatbelt assembly (includes pretensioner). Removed rust from the wiring harness connectors, dried them, and applied conductive grease and waterproof sealant. Cleared the fault codes; system self-check passed.
BYD DTC AI Analysis

Oxidized SRS module pins caused high resistance fault

✓
A 5-year-old Qin EV300 developed DTC B1773-00. Pretensioner resistance measured 2.1Ω (normal); harness continuity checked good. Further inspection of the SRS control module connector showed green corrosion on the right rear pretensioner pins, with contact resistance at 12Ω (out of spec). Cleaned the pins with electronic contact cleaner and a non-metallic brush, lightly sanded to restore the metallic shine, then applied specialized conductive grease. Fault code cleared; pretensioner circuit resistance returned to normal range.
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.