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 8

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.

The B1777 fault code on the Atto 8 indicates a short circuit in the left second-row seatbelt pretensioner circuit, with the Airbag Control Module reading 0 ohms instead of normal resistance. Severity is medium because it can keep the SRS warning light on and may affect restraint deployment in a crash. Common causes include moisture in the connector or damaged wiring under the seat/B-pillar. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Water ingress or moisture in the pretensioner connector causing a short circuit between terminals, commonly resulting from driving through water, leaking interior cleaning fluid, or aging floor sealing strips.
  • 2Chafed wiring harness under the seat or inside the B-pillar. Damaged wire insulation allows direct contact with the vehicle body metal, causing a short to ground.
  • 3Improper handling during seat removal and installation or interior modification causes an incompletely seated pretensioner connector, deformed terminals, or a broken connector latch resulting in a short circuit to the bracket.
  • 4Internal squib wire short circuit in the pretensioner unit (very rare; usually 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 via cross-validation).
What To Do
  • 1
    Use the BYD VDS2000 or a dedicated diagnostic tool to read fault codes. Confirm B1777 is a current (Active) fault, 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 fully discharge the SRS system. 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 signs of water ingress, green corrosion, loose connections, or deformed terminals.
  • 3
    Disconnect the pretensioner connector. Use a digital multimeter to measure the resistance between the two terminals of the pretensioner unit. The normal value is 1.8-3.0 Ω (refer to the specific vehicle workshop manual). If the resistance is close to 0 Ω, replace the pretensioner assembly (this typically 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 reading should be infinite (OL). If the resistance is 0 or close to 0, inspect for damage along the yellow wiring harness (dedicated SRS harness color), focusing on the seat slide rail interference area and the floor harness grommet.
  • 5
    Measure the resistance between the two terminals of the harness-side connector. The resistance should be infinite. Continuity indicates a short circuit between the positive and negative wires in the harness. Strip the harness sheath to locate and repair the damage. Replace the entire wiring harness if necessary.
  • 6
    After repairing or replacing damaged components, apply double insulation using electrical tape or heat-shrink tubing. Reconnect the connector and verify the locking tab engages fully. Reinstall the battery, clear the fault code, and perform the SRS system self-diagnostic cycle test (cycle the ignition switch ON-OFF 3 times). Verify DTC 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 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.