AU
B177D

DTC B177D indicates the airbag system (SRS) detects a 0 Ω circuit resistance in the right middle-row (typically second-row right) seat belt pretensioner — Seal U

Safety System

DTC B177D indicates the airbag system (SRS) detects a 0 Ω circuit resistance in the right middle-row (typically second-row right) seat belt pretensioner.

As a pyrotechnic actuator, the seat belt pretensioner squib normally has a resistance of 1.5-3.0 Ω (typically 2.0 Ω).

A 0 Ω resistance indicates a short circuit.

Potential causes include an internal pretensioner squib short, a wiring harness short to ground, or a short between connector terminals.

This fault causes the SRS ECU to determine the pretensioner is in an unsafe state.

During a collision, the pretensioner may fail to deploy and tension the seat belt, or the short circuit may force the airbag system into a degraded mode, compromising overall passive safety performance.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Harness wear under the seat: As the right middle-row seat slides forward and backward, the seat belt pretensioner harness rubs against the seat rail or body metal edge, damaging the insulation and causing a short to ground.
  • 2Connector water ingress or corrosion: Poor sealing of the pretensioner connector under the seat or at the lower B-pillar causes internal pins to short circuit after water wading, vehicle washing, or A/C condensate leakage.
  • 3Pretensioner internal squib short circuit: Manufacturing defects, aging, or severe impacts cause the pretensioner's internal bridge wire to short-circuit, dropping resistance to zero.
  • 4Improper repair procedures: directly measuring the pretensioner with a low-impedance multimeter (such as continuity buzzer mode), or the seat mechanism pinching a wiring harness improperly secured during previous repairs.
  • 5SRS ECU sampling circuit fault: A fault in the control unit's internal A/D converter or sampling resistor causes a false 0 Ω reading (relatively rare)
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (to fully discharge the SRS capacitor). Do not work on the airbag system with the ignition switch in the ON position.
  • 2
    Visual inspection: Check the pretensioner wiring harness under the right middle-row seat, on the inner side of the seat rail, and below the B-pillar for wear, cuts, or crush marks. Check the connector for looseness, water ingress, or signs of corrosion.
  • 3
    Isolation measurement: Disconnect the pretensioner connector and use a digital multimeter (set to low-current test mode) or a dedicated SRS tester to measure: ① the resistance between the two terminals of the pretensioner body (specified: 1.5–3.0 Ω; if 0 Ω, replace the pretensioner); ② the resistance to ground on the harness side (specified: infinite; if 0 Ω, the harness is short-circuited).
  • 4
    Wiring harness repair: If the wiring harness has a short circuit, repair the damaged insulation using heat-shrink tubing or tape. Reroute the harness to avoid the seat travel path. Replace the wiring harness assembly if necessary.
  • 5
    Component replacement: If the pretensioner body has a short circuit, replace the right middle-row seat belt pretensioner assembly (use genuine parts and match the resistance values of the new and old parts).
  • 6
    System verification: Restore connections, clear the fault codes, and perform an ignition cycle test. Use the diagnostic tool to read the live data stream to confirm the resistance has returned to normal (1.5-3.0Ω). Finally, perform an SRS system self-diagnosis to confirm no faults are present.
Real-World Cases
BYD DTC AI Analysis

Worn right second-row seat wiring harness caused short circuit in Song MAX

✓
A 2019 BYD Song MAX logged DTC B177D. Inspection found that when the right second-row seat was fully forward, the seatbelt pretensioner harness interfered with the metal edge of the seat rail. Prolonged friction wore through the insulation, exposing copper cores that contacted the body and shorted to ground. Repair: Repaired the damaged harness (3 wires, one completely severed), added corrugated tubing for protection, and repositioned the harness clips to eliminate interference throughout the full seat travel. After clearing the code, resistance returned to 2.1Ω.
BYD DTC AI Analysis

Tang DM-i pretensioner connector water ingress corrosion

✓
2021 Tang DM-i logged DTC B177D after the rainy season. Removed the right middle row seat and found green corrosion inside the pretensioner connector at the seat base. Moisture between the pins caused a micro-short; resistance measured near 0Ω. Root cause: Water entered when the vehicle waded through water above the door sill, or through a blocked sunroof drain tube. Repair: Cleaned the connector terminals, removed corrosion with electronic cleaner, applied conductive grease, and replaced the waterproof seal ring. Recommend checking the other seat connectors on the same side.
BYD DTC AI Analysis

Yuan EV Seatbelt Pretensioner Internal Squib Short Circuit Replacement Case

✓
2019 BYD Yuan EV set DTC B177D after collision repairs. The right second-row seatbelt pretensioner took a side impact without deploying. Disassembly revealed the pretensioner squib's bridgewire had deformed and shorted from the impact, measuring 0.2Ω. Pretensioners are single-use safety components; internal damage can cause electrical faults even without deployment. Solution: Replaced the right second-row seatbelt assembly (including pretensioner). New part measured 1.8Ω and the fault cleared. Check resistance on all seatbelt pretensioners when repairing accident-damaged vehicles.
BYD DTC AI Analysis

Qin Pro: Human-induced fault after repair

✓
2020 Qin Pro developed DTC B177D following replacement of the right-hand second-row seat trim. Inspection traced the fault to the technician pinching the pretensioner harness in the seat frame gap during seat removal and installation; this went unnoticed during reassembly, crushing the harness and shorting it to the frame. The technician had also used a standard multimeter on the resistance setting to directly measure the pretensioner—while not the direct cause, this presented a safety risk. Technicians freed the trapped harness, repaired the damaged section, dressed burrs on the seat frame, and fitted protective sleeving to the harness. Staff received training on SRS system operating standards.
BYD DTC AI Analysis

Multi-model SRS ECU software false positive fix

✓
Multiple BYD models (2021 Tang, 2019 Song MAX, etc.) have an algorithm defect in specific batches of SRS ECU software. Cold starts below -10°C trigger false B177D codes. Measured pretensioner resistance is 2.0Ω (normal); harness shows no shorts. VDS freeze frame data confirms the fault only occurs at cold start. Solution: Update SRS control unit software to the latest version (confirm version number for the specific model). Fault clears after upgrade. This is a software false fault; no hardware replacement needed.
Other Seal U 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.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.
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.