AU
B1772

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

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 — Atto 3
  • 1Loose right rear seat belt pretensioner connector (usually located under the seat or inside the C-pillar trim), water ingress and oxidation, or spread terminal pins 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 typically 2.0-3.0 Ω.
  • 3Prolonged bending and pinching of the under-seat wiring harness causes wire breakage or insulation damage (common in passenger vehicles with frequent fore-and-aft seat adjustment).
  • 4Pretensioner connector not fully seated or pinched wiring harness during vehicle modifications (such as installing seat covers or floor covering) or accident repairs.
  • 5SRS control unit (ACU) internal detection circuit fault causing false abnormal resistance readings (relatively rare; consider only after ruling out wiring issues)
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds for the SRS capacitor to fully discharge. Read and record the DTC freeze frame data. Confirm the fault frequency (current/history).
  • 2
    Visual inspection: Remove the right rear seat and lower C-pillar trim panel. Verify the pretensioner connector (usually yellow) is fully seated and check for 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 body 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 body 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 open circuits. Measure wiring harness insulation to ground and power to rule out short circuits. If wiring harness resistance is >1Ω 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 DTC B1772 does not return 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

✓
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

✓
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

✓
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 Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.