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B177A1B

DTC B177A1B indicates the SRS (Supplemental Restraint System/airbag system) control module detects an open circuit or resistance exceeding the normal threshold (typically >6Ω or infinite) in the left middle-row seat belt pretensioner circuit — Atto 3

Safety System

DTC B177A1B indicates the SRS (Supplemental Restraint System/airbag system) control module detects an open circuit or resistance exceeding the normal threshold (typically >6Ω or infinite) in the left middle-row seat belt pretensioner circuit.

The '1B' sub-code in the BYD diagnostic system typically indicates excessive circuit resistance or an open circuit, causing the ECU to determine the pretensioner is 'not present' or has lost communication.

This fault prevents the system from triggering the left middle-row seat belt pretensioner pyrotechnic device during a collision, increasing occupant forward travel and reducing crash protection.

The system illuminates the instrument cluster airbag fault warning light (SRS light) and may disable the deployment logic for the corresponding side airbag or curtain airbag.

3
Cases Logged
Real-World Cases
BYD DTC AI Analysis

BYD E2 left second-row pretensioner connector oxidised, causing open circuit

✓
The airbag warning light on the instrument panel remained illuminated. VDS showed DTC B177A1B (left middle row seatbelt pretensioner not present). Removed the left middle row seat and found the yellow pretensioner connector terminals severely oxidised due to previous water ingress; green copper corrosion caused infinite contact resistance. Cleaned the terminals with electronic contact cleaner, applied specialist conductive paste and waterproof silicone grease, then reconnected the connector. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Qin EV seat rail wear caused pretensioner harness open circuit

✓
Owner reported the airbag warning light illuminates intermittently on rough roads and clears on smooth surfaces. Retrieved DTC B177A1B as an intermittent fault. Inspection found the wiring harness protective sleeve damaged at the rear mounting point of the left second-row seat rail. When adjusting the seat fore-aft, the metal rail edge cut the harness, causing the internal copper wires to partially sever (intermittent continuity). Repaired the harness and rerouted the wiring, added rubber protective sleeves and securing clips to prevent contact with the rail. Fault fully resolved.
Original source ↗
BYD DTC AI Analysis

BYD E3 Airbag Control Module Spurious Fault

✓
No collision repair history. SRS warning light suddenly illuminated on the dash. Retrieved DTCs B177A1B (left middle row), B177B1B (right middle row), and other pretensioner faults. All pretensioner resistance measured 2.0Ω (normal); wiring harness continuity normal. Diagnosed as SRS ECU internal software bug causing false alarm. Performed SRS module software upgrade using VDS (V1.2 to V1.5), cleared DTCs. Fault has not recurred.
Original source ↗
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.