AU
B177F

DTC B177F indicates the SRS (Supplemental Restraint System) control module detects the Second Row Right Seatbelt Pretensioner circuit resistance exceeds the normal upper threshold (typically 2 — Seal U

Safety System

DTC B177F indicates the SRS (Supplemental Restraint System) control module detects the Second Row Right Seatbelt Pretensioner circuit resistance exceeds the normal upper threshold (typically 2.0-3.0 Ω, depending on the vehicle model).

As a key passive safety system actuator, the pretensioner contains a gas generator and an ignition coil.

Excessive resistance indicates high circuit resistance or a partial open circuit, which can cause the following: 1) The SRS cannot monitor the pretensioner readiness status. 2) The pretensioner may fail to deploy at the designed timing during a collision, compromising occupant restraint protection. 3) The airbag warning light remains illuminated, forcing the entire SRS into fail-safe mode and potentially preventing front and side airbag deployment.

This constitutes a hard or intermittent fault requiring immediate repair.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Poor pretensioner connector contact: Moisture, oxidation, or vibration increases terminal contact resistance in the connector located at the B-pillar or under the seat. This commonly occurs after vehicle wading or operation in high-humidity environments.
  • 2Seat belt pretensioner unit fault: Internal ignition coil open circuit or gas generator internal resistance wire aging causes resistance to rise from the standard 2.0Ω to above 3.5Ω.
  • 3Hidden open circuit in wiring harness: Frequent folding and movement break the internal copper wires in the right middle-row seat harness while the insulation remains intact, causing an intermittent connection.
  • 4SRS control module sampling circuit fault: Internal A/D conversion circuit or monitoring chip fault in the ACU (Airbag Control Unit) causing a false high resistance reading.
  • 5Improper repair or assembly: After replacing the seat or interior trim panel, the pretensioner connector fails to lock fully, or terminal pins back out, resulting in insufficient contact pressure.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (SRS capacitor discharge time). Use the diagnostic tool to confirm the fault code status is Current.
  • 2
    Visual inspection: Remove the right middle row seat side trim panel or lower B-pillar trim panel. Check that the pretensioner connector (usually a yellow plug) is fully seated and inspect for water ingress, corrosion, or terminal back-out.
  • 3
    Resistance measurement: Use a digital multimeter (0.1 Ω accuracy) to measure resistance directly at the pretensioner connector. Normal value: 1.8-2.2 Ω (at 20°C ambient). If resistance is >3.0 Ω, replace the pretensioner. If normal, proceed to the next step.
  • 4
    Harness continuity test: Disconnect the SRS control module connector. Measure harness continuity between the pretensioner connector and the ACU. Resistance must be <1 Ω. Also measure insulation resistance to ground and to the power supply. Resistance must be >1 MΩ.
  • 5
    Load test: Replace the pretensioner with a dedicated SRS test resistor (2Ω/0.25W). Clear the fault code, power on, and perform a self-check. If the fault code disappears, the pretensioner itself is faulty.
  • 6
    Repair and Verification: After replacing the faulty component or repairing the wiring harness, reconnect all connectors and restore the battery. Use the diagnostic tool to perform the 'SRS system self-test' and 'configuration verification', and confirm B177F becomes a history fault or no fault codes are present.
  • 7
    Function check: Perform seat folding and movement tests and simulate vibration. Confirm the fault does not recur and the airbag warning light turns off normally.
Real-World Cases
BYD DTC AI Analysis

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✓
2020 BYD Song MAX, 32,000 km, intermittent airbag warning light. Scan tool read code B177F (current). Found green corrosion inside the yellow wiring harness connector beneath the right second-row seat; suspected liquid spill from rear passengers. Cleaned terminals and applied conductive paste. Resistance dropped from 4.2Ω to 1.9Ω. Fault cleared. Recommend fitting waterproof protective sleeve.
BYD DTC AI Analysis

Tang DM seatbelt pretensioner connector not locked after seat replacement

✓
2021 Tang DM. Airbag warning light stayed on after accident repair. DTC B177F. Inspection found the right second-row seat was a new replacement assembly. The pretensioner connector was inserted but failed to lock—no 'click' heard and the plug pulled out easily. Reconnected the connector and confirmed the secondary lock engaged. Resistance read normal. Cleared the fault code; system returned to normal.
BYD DTC AI Analysis

Yuan EV pretensioner internal coil open circuit

✓
2019 BYD Yuan EV, 58,000 km, airbag warning light constantly illuminated. Resistance across right second-row pretensioner measured infinite (open circuit). Disassembled pretensioner and found internal igniter coil leads fractured. Replaced OE seatbelt assembly (including pretensioner), performed SRS coding and configuration matching, fault cleared. Cause: long-term frequent seat angle adjustment caused wiring harness fatigue.
BYD DTC AI Analysis

Qin Pro wiring harness chafing caused intermittent open circuit

✓
2020 Qin petrol. Airbag warning light on, DTC B177F. Visual inspection showed no faults. Resistance fluctuated between 2.0-8.0Ω when wiggling the harness. Removed B-pillar trim and found the harness insulation chafed at the metal bracket edge, with less than 30% of copper strands remaining. Repaired the wiring and fitted protective sleeving. Fault resolved.
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.