AU
B1777

This fault code indicates the Airbag Control Module (ACM) detects a 0 ohm circuit resistance in the left middle-row (second-row left) seatbelt pretensioner — Atto 3

Safety System

This fault code indicates the Airbag Control Module (ACM) detects a 0 ohm circuit resistance in the left middle-row (second-row left) seatbelt pretensioner.

In a normal SRS system, the pretensioner functions as a pyrotechnic actuator with a standard resistance typically between 1.8Ω and 3.0Ω (depending on the specific vehicle model).

A 0 ohm resistance indicates a hard short in the circuit, usually a short to ground or a short between terminals.

This causes the ACM to flag the pretensioner as failed.

This condition triggers the airbag system fail-safe mechanism, potentially preventing the corresponding airbag and related restraint systems from deploying during a collision.

The instrument cluster SRS warning light remains illuminated, posing a severe safety hazard.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Water ingress or moisture in the pretensioner connector causes a short circuit between terminals, commonly due to vehicle wading, leaking interior cleaning fluid, or aging floor sealing strips.
  • 2Wiring harness wear under the seat or inside the B-pillar damages the wire insulation, allowing direct contact with the vehicle body metal and causing a short to ground.
  • 3Improper handling during seat removal/installation or interior modification causes a partially seated pretensioner connector, deformed terminals, or a broken connector latch shorting against the bracket.
  • 4Pretensioner internal bridge wire short circuit (very rare; typically results from a manufacturing defect, abnormal voltage surge, or abnormal internal state after pretensioner deployment)
  • 5Airbag Control Module (ACM) internal detection circuit fault or damaged sampling resistor, causing a false resistance reading of 0 (rule out by cross-checking).
What To Do
  • 1
    Use the BYD VDS2000 or a dedicated diagnostic tool to read fault codes. Confirm B1777 is a current fault (Active), not a history fault. Record freeze frame data and check for related fault codes such as B17771A (resistance below threshold).
  • 2
    Disconnect the battery negative terminal and wait at least 3 minutes to ensure the SRS system fully discharges. Inspect the left second-row seat belt pretensioner connector (usually located under the left side of the seat or at the base of the B-pillar) for water ingress, green corrosion, looseness, or deformed terminals.
  • 3
    Disconnect the pretensioner connector. Use a digital multimeter to measure the resistance between the two terminals on the pretensioner body. The normal value is 1.8-3.0 Ω (refer to the specific model repair manual). If the resistance is close to 0 Ω, replace the pretensioner assembly (this usually requires replacing the seat belt).
  • 4
    Keep the pretensioner connector disconnected. Measure the resistance between both terminals on the harness-side connector and body ground (GND). The resistance must be infinite (OL). If the resistance is 0 or close to 0, inspect the yellow wiring harness (dedicated SRS harness color) for damage. Focus on the seat slide rail interference area and the floor harness pass-through grommet.
  • 5
    Measure the resistance between the two terminals of the harness-side connector. The value must be infinite. Continuity indicates a short circuit between the positive and negative wires. Strip back the harness sheath to locate and repair the damaged point. Replace the entire wiring harness if necessary.
  • 6
    After repairing or replacing the damaged components, apply double insulation using electrical tape or heat-shrink tubing. Reconnect the connector and ensure the locking tab fully engages. Reinstall the battery and clear the fault code. Perform the SRS system self-diagnostic cycle test (cycle the ignition switch ON-OFF 3 times). Confirm B1777 does not return and the SRS warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Water ingress in connector caused short circuit in flood-damaged vehicle

✓
A 2020 BYD Song MAX drove through floodwater to hub depth after heavy rain. The SRS warning light remained on at startup the next day. VDS2000 showed active fault code B1777. Removed the left middle-row seat trim panel and found standing water in the pretensioner connector. Copper terminals showed black oxidation and verdigris, causing a short between terminals. Cleaned the connector with electronic cleaner, blew it dry with compressed air, and measured normal pretensioner resistance at 2.1Ω. Reconnected the plug, applied insulating silicone grease, and wrapped for waterproofing. Fault cleared.
BYD DTC AI Analysis

Seat modification pinched and chafed the wiring harness.

✓
The owner of a 2019 Yuan EV retrofitted a seat ventilation system to the left middle row seat. During installation, the owner did not secure the loom with dedicated wiring harness clips, pinching the yellow pretensioner wiring against the edge of the seat rail bracket. As the seat moved back and forth, the insulation wore through and the internal conductors shorted to earth. Pretensioner body resistance measured normal, but the harness side read 0.3Ω to earth. Rerouted the wiring to clear the seat rail travel path, protected the loom with abrasion-resistant corrugated tubing, repaired the damaged conductors and resolved the fault.
BYD DTC AI Analysis

Connector contact fault after accident repair

✓
Following a minor collision on the left rear side, this 2021 Tang had the left rear door and B-pillar trim replaced at the dealership. One week after delivery, the customer reported the SRS warning light occasionally illuminating. Diagnostics showed intermittent B1777. Inspection revealed the pretensioner connector was not fully seated during reinstallation of the lower B-pillar trim, and the connector locking tab had broken. This let the connector contact the body metal bracket during driving vibrations, causing an intermittent short. Replaced the connector housing, re-crimped the terminals, and fully seated the connection. Fault has not recurred.
BYD DTC AI Analysis

Pretensioner internal short circuit failure

✓
2019 BYD Yuan, 80,000 km, no accident history. SRS warning light suddenly illuminated. Read current DTC B1777; clearing failed. Disconnected the connector and measured resistance across the pretensioner terminals: 0.1 Ω (near short). Diagnosed as internal igniter bridge wire insulation breakdown or moisture in the pyrotechnic element causing conductivity. Replaced the left middle row seatbelt pretensioner assembly (including seatbelt retractor). Performed Coding and airbag system calibration with the diagnostic tool. Fault resolved.
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.