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 — Qin Plus

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).

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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1Inter-layer short circuit in the pretensioner internal heating wire: Long-term 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 value.
  • 2Harness damaged and shorted to ground: The yellow SRS harness inside the B-pillar trim or under the seat (usually located in the left center door sill harness) shorts to body ground due to insulation damage from repeated bending, crushing, or rodent bites.
  • 3Connector water ingress and corrosion: Poor sealing of the left middle-row seat belt pretensioner connector (usually located below the B-pillar or at the seat frame) allows water entry after washing the vehicle 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 detected value is low).
What To Do
  • 1
    Safety Preparation: Disconnect the low-voltage battery negative terminal and wait at least 90 seconds (to fully discharge the SRS capacitor). Remove the service disconnect plug (execute 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 a 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 matter. 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: 2.0-5.0Ω. If <1.5Ω, replace the pretensioner assembly. If normal, check the wiring.
  • 5
    Harness continuity check: Measure the resistance to ground at the pretensioner connector harness side (ECU side). Normal resistance is 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 hole.
  • 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: model and software version must match).
  • 7
    Component replacement: When replacing the pretensioner, use genuine BYD parts (the 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 Setting' (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 Qin Plus 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.