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 8

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 Atto 8 indicates low resistance in the left second-row seat belt pretensioner circuit. This is a medium-severity SRS fault that may trigger the airbag warning light and affect crash protection. Common causes include a shorted pretensioner heating wire or damaged SRS wiring shorting to ground. Have an authorised dealer inspect and repair the system. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Short circuit between pretensioner heating wire layers: 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.
  • 2Wiring harness damaged and shorted to ground: Repeated bending, crushing, or rodent bites damage the insulation of the yellow SRS wiring harness (inside the B-pillar trim or under the seat, typically within the left center door sill harness), 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 terminals.
  • 4Abnormal terminal contact: Connector terminal back-out, deformation, or foreign object ingress causes abnormal conduction, 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 measured value is low).
What To Do
  • 1
    Safety preparation: Disconnect the low-voltage battery negative terminal, wait at least 90 seconds (allow the SRS capacitor to fully discharge), and remove the service disconnect plug (execute the high-voltage power-down procedure for new energy vehicles).
  • 2
    Fault Confirmation: Use VDS2000 or a dedicated BYD diagnostic tool to read fault codes. Confirm B1778 is a current fault (Active). Record freeze frame data. Check for accompanying fault codes (e.g., 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 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 at the harness side (ECU side) of the pretensioner connector. 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 harness pass-through location at the door sill.
  • 6
    ECU verification: If the pretensioner and wiring harness are normal, use an oscilloscope to check the sampling voltage waveform on SRS ECU channel X (left middle-row pretensioner drive pin), or verify the ECU by direct substitution (Note: Requires same model and matching software version).
  • 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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.