AU
B1772

DTC B1772 indicates the right rear seatbelt pretensioner ignition circuit resistance exceeds the standard upper limit (typically >3 — Seal U

Safety System

DTC B1772 indicates the right rear seatbelt pretensioner ignition circuit resistance exceeds the standard upper limit (typically >3.6Ω or open circuit).

The seatbelt pretensioner is a key SRS (Supplemental Restraint System) actuator containing an internal squib.

During a collision, the SRS control unit (ACU) sends current to the squib to ignite the propellant, instantly tightening the seatbelt to protect the occupant.

Excessive resistance indicates high resistance, poor contact, or a complete open circuit in the ignition circuit, preventing the ACU from activating the pretensioner.

During a collision, the rear occupant on that side loses pretensioner protection, significantly increasing safety risks.

Typical causes include connector oxidation, wiring harness damage, or internal pretensioner squib failure.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Loose right rear seat belt pretensioner connector (usually located under the seat or inside the C-pillar trim), oxidation due to water ingress, or spread terminals causing increased contact resistance.
  • 2Abnormally high resistance or open circuit in the pretensioner internal igniter (blown internal bridge wire, damp or aged propellant). Standard resistance is usually 2.0-3.0Ω.
  • 3Prolonged bending and compression of the under-seat wiring harness causes broken wires or damaged insulation (common in passenger vehicles with frequent fore-and-aft seat adjustment).
  • 4Pretensioner connector not fully connected or wiring harness pinched during vehicle modifications (such as installing seat covers or floor mats) or accident repairs.
  • 5Internal fault in the SRS control unit (ACU) detection circuit causing a false abnormal resistance reading (relatively rare; consider after ruling out wiring faults).
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds to completely discharge the SRS capacitor. Read and record the DTC freeze frame data and confirm the fault frequency (current/history).
  • 2
    Visual inspection: Remove the right rear seat and lower C-pillar trim panel. Check that the pretensioner connector (usually yellow) is fully seated and free of water ingress, corrosion, or foreign objects. Inspect the wiring harness corrugated conduit under the seat for damage. Check the wires for crush marks from the seat rails.
  • 3
    Resistance measurement: Disconnect the pretensioner connector. Use a digital multimeter to measure the resistance on the pretensioner unit side (standard value: 2.0 ± 0.2 Ω; refer to the workshop manual for details). If the resistance is >3.6 Ω or infinite, the pretensioner unit is faulty. Replace the seat belt assembly (the pretensioner is a non-repairable part).
  • 4
    Wiring harness inspection: Measure continuity from the harness side of the connector to the SRS control unit to check for an open circuit. Measure harness insulation to ground and power to rule out a short circuit. If harness resistance is >1 Ω or a short circuit exists, repair or replace the wiring harness.
  • 5
    System reset: After repair, reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code and perform SRS system self-learning (required on some models). Perform a road test or simulation test to confirm B1772 does not reappear and the airbag warning light turns off normally.
Real-World Cases
BYD DTC AI Analysis

Oxidized right rear connector on BYD Song MAX causing high resistance

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Symptoms: Dashboard airbag warning light constantly illuminated; scan tool reporting DTC B1772. Diagnosis: Removed the right rear seat. Found obvious green corrosion inside the seatbelt pretensioner connector (yellow) with poor pin contact. Measured pretensioner body resistance at 2.1Ω (normal), but circuit resistance read 5.8Ω with the connector attached. Repair: Cleaned the connector pins with precision electrical contact cleaner, applied conductive protectant, and reinserted the connector firmly. Fault cleared.
BYD DTC AI Analysis

Seat rail crushed wiring harness causing B1772 in BYD Tang DM

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Symptoms: Airbag warning light illuminates intermittently on rough roads. Fault code B1772 (intermittent). Diagnosis: Found the seatbelt pretensioner harness beneath the right rear seat crushed by the seat rails. Damaged insulation caused internal wires to partially fracture (intermittent contact). Repair: Cut out the damaged section, soldered the wires, and sealed with heat-shrink tubing. Rerouted the harness to avoid interference with moving parts. Fault completely resolved.
BYD DTC AI Analysis

BYD Yuan EV: Replaced pretensioner assembly due to internal open circuit

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Symptoms: Airbag warning light stays on continuously; only current DTC B1772 present. Diagnosis: Disconnected the pretensioner connector and measured resistance directly across the pretensioner terminals—infinite resistance (open circuit). Wiring harness continuity and power supply both normal. Confirmed pretensioner internal igniter bridge wire blown or propellant failure. Resolution: Replaced the right rear seatbelt assembly (includes pretensioner) because the pretensioner is integrated with the seatbelt retractor. After installation, coded and configured the unit with a diagnostic tool. Cleared fault codes; system normal.
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.