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 — Seal 6 EV

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).

The B1778 fault code on the Seal 6 EV indicates low resistance in the left second-row seat belt pretensioner circuit, which may trigger the airbag warning light and affect SRS protection. Severity is medium. Common causes include an internal short in the pretensioner heating wire or damaged yellow SRS wiring shorted to ground. The recommended fix is to visit an authorised dealer for diagnosis and safe repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Seal 6 EV
  • 1Inter-layer short circuit in the pretensioner internal heating wire: 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: The yellow SRS wiring harness inside the B-pillar trim or under the seat (usually located in the left center door sill harness) has insulation damage from repeated bending, crushing, or rodent chewing, causing a short to body ground.
  • 3Connector water ingress and corrosion: Poor sealing of the left middle-row seat belt pretensioner connector (usually located below the B-pillar or on the seat frame) allows water ingress after car washing or wading, causing a micro-short circuit between the terminals.
  • 4Abnormal terminal contact: Connector terminal back-out, deformation, or foreign material ingress causes abnormal continuity, creating a low-resistance path.
  • 5SRS ECU sampling circuit fault: Damage to the airbag control module's internal A/D conversion circuit or shunt resistor causes a false low resistance reading (actual resistance is normal but the detected value is low).
What To Do
  • 1
    Safety preparation: Disconnect the low-voltage battery negative terminal, wait at least 90 seconds (to ensure the SRS capacitor fully discharges), and 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 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. Check the pretensioner connector (part number usually BYD-SRS-02-L) for looseness, signs of water ingress, burn marks, or foreign matter. Check 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 of 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: Infinity. If continuity exists, inspect the harness in sections. Focus on the harness bend at the B-pillar hinge and the sill harness pass-through location.
  • 6
    ECU verification: If the pretensioner and wiring harness are normal, use an oscilloscope to check the sampled voltage waveform at SRS ECU channel X (left middle row pretensioner drive pin), or verify the ECU by direct replacement (Note: The model and software version must match).
  • 7
    Component replacement: When replacing the pretensioner, use only genuine BYD parts (pyrotechnic charges vary by model). During installation, fully engage the connector locking tab and route the wiring harness with appropriate slack (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

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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

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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 Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV overview →
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.