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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
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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.