AU
B179A-00

This DTC indicates the Airbag Control Unit (ACU) detects an open or disconnected circuit in the front passenger side second stage seat belt pretensioner — Seal U

Safety System

This DTC indicates the Airbag Control Unit (ACU) detects an open or disconnected circuit in the front passenger side second stage seat belt pretensioner.

The second stage pretensioner is a critical component of the restraint system.

During severe collisions, it works with the first stage, using an igniter to detonate the gas generator and rapidly tighten the seat belt.

The ACU continuously monitors this circuit's resistance (normal range: approx. 1.8-3.2Ω).

When the ACU detects infinite resistance or a value exceeding the threshold, it determines a disconnected fault and logs B179A-00.

This fault prevents the pretensioner from deploying during a collision, reducing chest and head restraint protection for the front passenger.

It also illuminates the SRS warning light continuously.

Some models may enter a safety protection mode, restricting the normal deployment logic of other airbags.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Front passenger seat belt pretensioner electrical connector loose or has poor contact (commonly caused by failing to fully seat the connector after B-pillar trim panel removal/installation, window tinting, or modifications)
  • 2Open circuit in the pretensioner body internal igniter or gas generator failure (internal open circuit, possibly due to aging or damage from a minor collision without deployment)
  • 3Open circuit, wear, or short to body ground in the wiring harness between the ACU and pretensioner (especially fatigue breakage in the B-pillar hinge area from repeated door opening and closing).
  • 4Connector terminal oxidation, corrosion, or water ingress (common after driving through water, car wash fluid seepage, or long-term parking in high-humidity environments)
  • 5Airbag Control Unit (ACU) internal detection circuit fault or software calibration error (damaged hardware sampling circuit or false reading)
What To Do
  • 1
    Use a dedicated diagnostic tool (BYD ED400 or Launch X431) to read the fault code, confirm B179A-00 is a current fault (Active), and record the ambient temperature and vehicle status from the freeze frame data.
  • 2
    Perform the safe power-down procedure: Turn off the ignition switch, disconnect the negative battery terminal, and wrap it with insulating tape. Wait at least 3 minutes to ensure the SRS energy storage capacitor fully discharges, preventing accidental airbag deployment.
  • 3
    Remove the front passenger lower B-pillar trim panel (requires removing the seat belt height adjuster retaining bolt). Locate the yellow second-stage pretensioner connector (usually equipped with a two-way short-circuit protection tab). Verify the connector fully seats and locks; listen for a 'click' to confirm locking.
  • 4
    Disconnect the pretensioner connector (Note: The short-circuit protection plate automatically shorts both terminals upon disconnection; this is normal). Use a multimeter to measure the resistance between the pretensioner unit terminals. The standard value is 1.8-3.2Ω. If the resistance is infinite or zero, the pretensioner unit is faulty.
  • 5
    Check wiring harness continuity: Measure continuity between the pretensioner connector and the corresponding ACU pin (usually located under the center console or central tunnel). Inspect the wiring harness in the B-pillar area for breaks, wear, or short circuits to the body. Pay special attention to interference points between the harness and the B-pillar hinge.
  • 6
    Clean the connector terminals using dedicated electronic contact cleaner to remove oxidation. Apply dedicated conductive grease if necessary. Reconnect the connector and secure the wiring harness. Ensure the harness routing has adequate slack and does not contact sharp edges.
  • 7
    Reconnect the battery negative terminal, clear the fault code, and turn the ignition switch to the ON position (do not start). Observe whether the SRS warning light turns off after the self-check (turning off after approximately 6 seconds is normal; if it remains illuminated, re-inspect).
  • 8
    Perform an SRS system function test: Use the diagnostic tool to perform a 'Seat Belt Pretensioner' actuator test (if the diagnostic tool supports it), or perform a simulated crash signal test. Confirm the ACU correctly detects the pretensioner resistance and the fault is completely cleared.
Real-World Cases
BYD DTC AI Analysis

Qin EV450: Pretensioner connector loose after B-pillar trim panel removal and installation

✓
A 2018 Qin EV450 showed a constant SRS warning light after window tinting and interior modification. ED400 read DTC B179A-00 (current fault) with no other codes present. The technician removed the passenger-side B-pillar trim and found the second-stage pretensioner yellow connector only about halfway inserted, not fully seated, causing poor terminal contact. The technician reseated the connector until it clicked, secured the harness with a cable tie to prevent loosening, cleared the code, and the SRS light went out. Road test confirmed no recurrence.
BYD DTC AI Analysis

Water exposure corroded the Qin 100 pretensioner connector, causing an open circuit.

✓
A 2017 Qin 100 was driven through approximately 40 cm of water during heavy rain. The SRS light illuminated the next day on startup. The scan tool showed codes B179A-00 and B179B-00 (power short circuit) alternating. Removed the passenger seat and B-pillar trim; the pre-tensioner connector had obvious water stains and green copper corrosion with severely oxidized terminals. Cleaned the connector thoroughly with electronic cleaner, blew it dry, and sanded the terminals with fine sandpaper. Resistance measured normal. Reassembled and cleared the fault codes. Advised the customer to replace the connector waterproof seal and check for standing water under the carpet.
BYD DTC AI Analysis

BYD Qin EV300: Replaced seatbelt assembly due to internal open circuit in pretensioner

✓
A 2017 BYD Qin EV300 with no collision history: SRS warning light suddenly illuminated while driving. Scanned and found active code B179A-00. Voltage at the pretensioner connector measured normal (approximately 12V). Disconnected the plug and measured infinite resistance (open circuit) directly at the pretensioner terminals — diagnosed as internal squib open circuit, likely an early batch part defect. Replaced the passenger side seat belt assembly (includes pretensioner, P/N: SA-58510-XXXX). Post-installation resistance measured 2.1Ω. Cleared fault codes and performed SRS system configuration coding using ED400 ('Replace ECU/Part' function). Fault resolved.
BYD DTC AI Analysis

Chafed B-pillar wiring harness causes intermittent B179A-00 fault

✓
Qin 80 owner reported the SRS warning light occasionally comes on, especially over speed bumps or rough roads. Scanner showed intermittent B179A-00. Inspection found the B-pillar hinge wiring harness retaining clip had detached, allowing the harness to chafe against a sharp metal edge. Long-term abrasion damaged the insulation and partially fractured the copper core without fully severing it; driving vibration caused an intermittent open. Cut out the damaged section, soldered the wires, and insulated with heat-shrink tubing. Rerouted the harness, added a rubber sleeve, and resecured the clip. Road-tested 50 km continuously; the fault did not recur.
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.