AU
B178211

This DTC indicates the SRS (Supplemental Restraint System) ECU detected a short to ground in the right second-row seat belt pretensioner ignition circuit — Seal U

Safety System

This DTC indicates the SRS (Supplemental Restraint System) ECU detected a short to ground in the right second-row seat belt pretensioner ignition circuit.

As a pyrotechnic safety device, the pretensioner typically has a resistance of 1.5-3.0Ω.

The ECU determines component status by monitoring circuit current and resistance.

The ECU registers a short to ground if circuit resistance falls below the threshold (typically <0.8Ω) or insulation resistance to ground drops abnormally.

This fault immediately forces the SRS into fail-safe mode, disables deployment of the affected seat pretensioner and potentially associated airbags, and illuminates the instrument panel airbag warning light.

Because the pretensioner uses a squib as the actuating element, a short to ground can cause unintended ignition energy leakage or system misjudgment, creating a potential safety hazard.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Worn or split wiring harness sleeve under the right middle-row seat or at the B-pillar allows the ignition circuit wire to directly contact the vehicle body metal frame, causing a short circuit.
  • 2Pretensioner connector (usually located under the seat or inside the B-pillar trim, marked in yellow): Seal failure allows water ingress after wading or washing the vehicle, causing terminal corrosion and a short circuit.
  • 3During accident repairs, improper routing of the pretensioner wiring harness allows the seat slide rail to pinch the harness, or a detached retaining clip leads to long-term chafing that damages the insulation.
  • 4Pretensioner assembly internal igniter fault or internal short circuit (less common; usually exhibits obvious external damage or collision history)
  • 5SRS ECU internal monitoring circuit fault causing false short circuit detection (rule out via cross-checking)
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS or ED400) to read complete DTCs. Confirm whether B178211 is a current (Active) or history fault. Check the right second-row pretensioner resistance in the SRS data stream (normal range: 1.5-3.0 Ω).
  • 2
    Disconnect the low-voltage battery negative terminal and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 3
    Remove the right middle-row seat (if applicable) or right B-pillar lower trim panel. Locate the pretensioner connector (usually a yellow double-lock connector with shorting bar protection).
  • 4
    Disconnect the pretensioner connector. Use a multimeter to measure the resistance between the two terminals on the pretensioner side (normal: 1.5–3.0 Ω) and the resistance from each terminal to ground (>1 MΩ) to determine whether the fault is in the pretensioner assembly or the wiring harness.
  • 5
    If the pretensioner is normal, check the wiring harness side: measure the wiring harness insulation to ground between the connector and the SRS ECU. Focus on areas near the seat slide rail, floor harness bends, and points contacting metal parts.
  • 6
    If the wiring harness is damaged, repair it using heat-shrink tubing or high-voltage insulating tape, and re-secure it at a safe distance from the vehicle body metal. If the pretensioner has an internal short circuit, replace the assembly (non-repairable).
  • 7
    After repair, reconnect all components and restore power. Use the diagnostic tool to clear the DTC and perform an SRS system self-test (required conditions: vehicle stationary, normal battery voltage). Confirm the fault is resolved and the warning lamp is off.
Real-World Cases
BYD DTC AI Analysis

BYD E3 right middle-row pretensioner short circuit after water ingress

✓
A 2020 BYD E3 developed a continuous airbag warning light after driving through deep water in heavy rain. DTC B178211 was logged. Inspection revealed obvious water stains on the right side of the carpet. After removing the right rear seat, we found water inside the pretensioner connector (located at the crossmember beneath the seat) and green corrosion on the terminals. We cleaned the connector and dried it with compressed air; the pretensioner itself tested normal, but the harness side showed only 0.3Ω resistance to ground. Further inspection revealed the connector seal had deteriorated and was missing, allowing water ingress during wading. We replaced the harness connector and resealed it, which resolved the fault.
BYD DTC AI Analysis

Qin EV Seat Adjustment Causes Wiring Harness Wear Case

✓
2019 Qin EV owner reported the airbag warning light coming on intermittently at specific seat positions. Read DTC B178211 (intermittent fault). Inspection found the right second-row seat manually adjustable. At fully forward and rearward positions, the seat rails pinch the pretensioner harness (running from the B-pillar to the seat). Long-term friction wore through the insulation, exposing the copper core to the seat frame and creating a short to ground. Re-routed the wiring and secured it with dedicated harness retention clips, ensuring the loom stays clear throughout the full seat travel range. Fault fully resolved.
BYD DTC AI Analysis

E2: Pretensioner wiring not secured after accident repair

✓
A 2019 E2 had the SRS warning light stay on after right rear-side collision repairs. DTC B178211. Inspection found technicians had replaced the right second-row seatbelt assembly during the accident repair but failed to properly clip the pretensioner wiring harness into the B-pillar harness bracket. The harness loosened and chafed against the body metal edge, damaging the insulation and causing a short to ground. Repaired the damaged harness, resecured the harness clip to the specified torque and position, cleared the fault code, and the system returned to normal.
BYD DTC AI Analysis

Rare internal short circuit in pretensioner body

✓
A 2020 E3's airbag warning light suddenly illuminated during normal use. The vehicle had no accident or water ingress history. The diagnostic tool showed active DTC B178211. Disconnecting the pretensioner connector revealed 0.2Ω between terminals (well below normal) and continuity to ground on both terminals. This indicated an internal pretensioner igniter short. Replaced the right middle row seatbelt pretensioner assembly (requires replacing the complete seatbelt retractor assembly). Fault cleared. Likely caused by a batch defect in the internal pretensioner components.
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.