AU
B1778

DTC B1778 indicates the left middle-row (second-row left) seat belt pretensioner resistance falls below the normal threshold set by the SRS ECU — Atto 3

Safety System

DTC B1778 indicates the left middle-row (second-row left) seat belt pretensioner resistance falls below the normal threshold set by the SRS ECU.

Standard resistance is typically 2.0–5.0 Ω; the ECU sets the fault upon detecting <1.5 Ω or near 0 Ω.

The pretensioner is a pyrotechnic safety device containing an internal heating wire and pyrotechnic charge.

During a collision, the ECU triggers deployment to rapidly tighten the seat belt.

Low resistance typically indicates a circuit short to ground, an internal short in the pretensioner heating wire, or a grounded connector terminal.

Consequently, the SRS system flags a risk of unintended deployment or functional failure, illuminates the airbag warning light, and may disable related crash protection functions.

On BYD e-Platform and DM models, this fault may also affect the vehicle high-voltage interlock logic (some models link seat belt status to high-voltage readiness).

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Pretensioner internal heating wire interlayer short circuit: Prolonged exposure to damp environments or manufacturing defects causes the internal pyrotechnic charge to absorb moisture, damaging the heating wire insulation and abnormally lowering the resistance.
  • 2Harness damaged and shorted to ground: Repeated bending, crushing, or rodent bites damage the insulation on the yellow SRS harness inside the B-pillar trim or under the seat (usually located in the left center door sill harness), causing a short to body ground.
  • 3Connector water ingress and corrosion: Poor sealing of the left second-row seat belt pretensioner connector (usually located below the B-pillar or on the seat frame) allows water entry after car washing or wading, causing a micro-short circuit between terminals.
  • 4Abnormal terminal contact: Connector terminal back-out, deformation, or foreign object ingress causes abnormal continuity, creating a low-resistance path.
  • 5SRS ECU sampling circuit fault: A damaged internal A/D conversion circuit or shunt resistor in the airbag control module causes a false low resistance reading (actual resistance is normal but measured value is low).
What To Do
  • 1
    Safety preparation: Disconnect the low-voltage battery negative terminal and wait at least 90 seconds (allow the SRS capacitor to discharge completely). Remove the service disconnect plug (perform the high-voltage power-down procedure for new energy vehicles).
  • 2
    Fault confirmation: Use VDS2000 or a BYD dedicated diagnostic tool to read fault codes. Confirm B1778 is the current fault (Active). Record freeze frame data and check for accompanying fault codes (such as B17771A).
  • 3
    Visual inspection: Remove the left B-pillar lower trim panel. Inspect the pretensioner connector (part number usually BYD-SRS-02-L) for looseness, water ingress, burning, or foreign objects. Inspect the wiring harness corrugated conduit for damage.
  • 4
    Resistance measurement: Disconnect the pretensioner connector. Use a digital multimeter (resistance setting) to measure the resistance directly between the two terminals on the pretensioner body. Standard value: 2.0-5.0 Ω. If <1.5 Ω, replace the pretensioner assembly. If normal, check the wiring.
  • 5
    Harness continuity check: Measure the resistance to ground on the harness side (ECU side) of the pretensioner connector. Normal value: Infinite. If continuity exists, inspect the harness section by section. Focus on the harness bend point at the B-pillar hinge and the sill harness pass-through position.
  • 6
    ECU verification: If the pretensioner and wiring harness are normal, use an oscilloscope to check the sampled voltage waveform on SRS ECU channel X (left middle-row pretensioner drive pin), or verify the ECU by direct replacement (Note: same model and matching software version required).
  • 7
    Component replacement: When replacing the pretensioner, use genuine BYD parts (pyrotechnic charge varies by model). During installation, fully engage the connector locking tab and leave appropriate slack in the wiring harness (neither too tight nor too loose).
  • 8
    System reset: Connect the battery, use the diagnostic tool to clear the fault codes, and perform 'SRS System Self-learning' or 'Configuration Settings' (some models require rewriting the pretensioner resistance parameters). Confirm the warning light turns off and the system has no fault codes.
Real-World Cases
BYD DTC AI Analysis

Song MAX Left Second-Row Pretensioner Water Ingress Short Circuit Case

✓
2019 BYD Song MAX DM, 32,000 km. Airbag warning light on. Code: B1778. History of sunroof drain blockage caused water to accumulate below the left B-pillar. Removed left second-row seat belt lower anchor trim panel; found water stains in the pretensioner connector and oxidised, blackened terminals. Pretensioner resistance: 2.3 Ω (normal). Harness-side connector to ground: 0.8 Ω. Cleaned and dried the connector; insulation restored. Reassembled and sealed with adhesive. Fault resolved.
BYD DTC AI Analysis

Yuan EV wiring harness pinch caused low resistance

✓
2019 BYD Yuan EV535. DTC B1778 triggered after accident repairs. Left rear door impact damage; left B-pillar trim panel replaced during repairs. Found left second-row pretensioner harness pinched at B-pillar trim clip, insulation damaged and copper wire shorting to earth. Pretensioner measured normal at 2.8 ohms, but harness showed continuity to earth. Cut out damaged section, soldered wires and sealed with heat-shrink tubing. Rerouted harness to avoid trim clip interference. Cleared fault codes; system normal.
BYD DTC AI Analysis

Tang DM Pretensioner Internal Short Circuit Replacement Case

✓
2021 BYD Tang DM, 50,000 km, no accident history. Intermittent airbag warning light. DTC B1778 (intermittent). Checked connectors — no abnormalities. Pretensioner resistance measured 0.5–1.2 Ω (unstable), indicating internal heating wire short. Replaced left middle row seatbelt pretensioner assembly (P/N: BYD-SRS-PT-02L); new part measured 2.4 Ω. Configured SRS system after installation; fault cleared. Root cause: pretensioner internal propellant moisture caused heating wire corrosion.
BYD DTC AI Analysis

Aftermarket seat covers caused fault in Qin Pro

✓
2020 Qin Pro petrol. Customer reported airbag warning light on. Scanned DTC B1778. Inspection found customer recently installed full seat covers; fixing clips pierced the SRS wiring harness under the left middle-row seat, causing the pretensioner circuit to short to ground. Repaired damaged harness (3 wires including pretensioner high and low potential lines), adjusted harness routing and added protective sleeve. Advised customer to avoid sharp fasteners near SRS components.
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.