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 — Seal 6 EV

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 Seal 6 EV indicates a short circuit in the left second-row seatbelt pretensioner circuit, with the Airbag Control Module detecting 0 ohms resistance. This is a medium-severity safety issue because it can disable related restraint functions and keep the SRS warning light on. 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 — Seal 6 EV
  • 1Water ingress or moisture in the pretensioner connector causing a short circuit between terminals, commonly due to driving through water, interior cleaning fluid leakage, or floor sealing strip aging.
  • 2Chafed wiring harness under the seat or inside the B-pillar. Damaged insulation allows the wire to directly contact the vehicle body metal, causing a short to ground.
  • 3Improper handling during seat removal/installation or interior modification causes an unseated pretensioner connector, deformed terminals, or a broken connector clip shorting against the bracket.
  • 4Internal bridge wire short circuit in the pretensioner body (very rare; usually caused by manufacturing defects, abnormal voltage surges, or abnormal internal conditions following 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-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 historical fault. Record freeze frame data and check for related fault codes such as B17771A (resistance below threshold).
  • 2
    Disconnect the battery negative terminal. Wait at least 3 minutes for the SRS system to fully discharge. Inspect the left second-row seat belt pretensioner connector (typically 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 and use a digital multimeter to measure the resistance between the two terminals of the pretensioner body. The normal value should be 1.8–3.0 Ω (refer to the specific model repair manual). If the resistance is close to 0 Ω, replace the pretensioner assembly (usually requires replacing the seat belt).
  • 4
    Leave the pretensioner connector disconnected. Measure the resistance from each terminal of the harness-side connector to body ground (GND). The resistance must be infinite (OL). If the resistance is 0 or near 0, inspect the yellow wiring harness (dedicated SRS harness color) for damage, focusing on the seat slide rail interference area and the floor harness pass-through grommet.
  • 5
    Measure the resistance between the two terminals on the harness-side connector; the value must be infinite. If continuity exists, this indicates a short circuit between the positive and negative wires in the harness. Strip back the harness sheath to locate and repair the damaged point. Replace the entire harness section if necessary.
  • 6
    After repairing or replacing damaged components, apply double insulation using electrical tape or heat-shrink tubing. Reconnect the connector and fully engage the locking tab. Reinstall the battery, clear the fault code, and perform the SRS 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 Seal 6 EV Fault Codes
B1108For 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.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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.