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B178F11

DTC B178F11 indicates a short to body ground in the front passenger dual-stage airbag (Stage 2) ignition circuit — Seal U

Safety System

DTC B178F11 indicates a short to body ground in the front passenger dual-stage airbag (Stage 2) ignition circuit.

In the BYD SRS system, the airbag utilizes a dual-stage ignition design: low speeds trigger the first stage, and high speeds trigger the second stage.

The '11' suffix indicates the control unit detects circuit resistance below the threshold (typically <0.8Ω), determining a short to ground.

This safety-critical fault prevents the SRS system from deploying the second-stage airbag during a collision or creates a risk of unintended deployment.

The fault can originate in the clock spring, instrument panel harness, floor harness, or inside the airbag module.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Worn front passenger airbag wiring harness insulation causes contact with vehicle body metal, commonly due to improper harness securing after dashboard removal and installation.
  • 2Internal short circuit in the airbag clock spring (spiral cable) caused by long-term steering wheel rotation, which breaks the internal wire and shorts it to ground.
  • 3Water ingress and oxidation in the SRS system connector. Poor sealing, especially below the A-pillar or where the floor wiring harness passes through the firewall, causes corrosion and a short circuit.
  • 4Front passenger airbag module internal igniter short to housing, typically resulting from moisture in the airbag module or a manufacturing defect.
  • 5SRS control unit internal driver circuit fault causing a false short-to-ground report (confirm after ruling out external wiring).
What To Do
  • 1
    Perform high-voltage system power-down: Disconnect the 12V battery negative terminal and wait at least 3 minutes for the SRS capacitor to fully discharge. While waiting, press the horn button to release residual charge.
  • 2
    Read freeze frame data: Use the VDS2000/Launch X431 diagnostic tool to record ambient temperature, vehicle speed, and crash sensor status at the time of the fault, and confirm if the fault is intermittent.
  • 3
    Visual inspection: Remove the front passenger glove box. Inspect the SRS wiring harness for obvious damage or crushing. Specifically check the yellow dedicated connector (usually marked F201) for bent pins or water ingress.
  • 4
    Resistance measurement: Disconnect the airbag module connector. Use a digital multimeter to measure the resistance to ground on the wiring harness side. Normal resistance is infinite. If the resistance is <1Ω, inspect the wiring harness in sections, focusing on the clock spring (if applicable) and the instrument panel wiring harness.
  • 5
    Airbag module inspection: Substitute the airbag with the dedicated load tool. If the fault code changes to open circuit (Open), replace the front passenger airbag module. If the fault code remains a short circuit, continue inspecting the wiring harness.
  • 6
    Wiring harness repair: After locating the short circuit, repair it using high-temperature tape (Tesa 51036 or equivalent). Maintain a >20 mm clearance between the wiring harness and the metal frame. Install corrugated conduit for protection if necessary.
  • 7
    System verification: Reconnect all connectors, restore power, and use the diagnostic tool to perform the 'SRS system self-test' and 'collision simulation test' (without deployment). Confirm B178F11 changes to a history fault, then clear the code.
  • 8
    Road test verification: Perform a road test of over 5 km, including bumpy road sections. Confirm the fault does not recur and verify the SRS warning light turns off normally after self-check.
Real-World Cases
BYD DTC AI Analysis

E2 dashboard modification damaged wiring harness

✓
2019 BYD E2, 32,000 km. DTC B178F11 appeared after installing a large-screen navigation unit in the dashboard. Inspection found screws used to secure the dashboard during the modification had penetrated the SRS wiring harness sheath, shorting the passenger-side secondary airbag circuit to the dashboard frame. Repaired and rerouted the harness; fault cleared. Recommendation: Avoid yellow SRS harness channels during dashboard modifications.
BYD DTC AI Analysis

Qin EV clock spring internal short circuit

✓
2019 BYD Qin EV. DTC B178F11 set following accident repairs. The SRS warning light illuminated when turning the steering wheel to specific angles. Testing revealed intermittent continuity between pins 5–6 of the clock spring (spiral cable) and the housing. Replaced the OEM clock spring (part number: DK-34792), then performed steering angle sensor calibration and SRS configuration using a scan tool. Fault resolved completely.
BYD DTC AI Analysis

E3: A-pillar water leak caused connector corrosion

✓
2020 E3, used in southern regions. DTC B178F11 occurred frequently during rainy season. Inspection revealed green corrosion inside the SRS connector (yellow plug) below the A-pillar. Seal ring deterioration allowed water ingress. Cleaned the connector pins, applied conductive paste, replaced the seal ring and applied adhesive sealant. Also cleared the blocked sunroof drain to prevent further water ingress.
BYD DTC AI Analysis

False positive for internal short circuit in airbag module

✓
2019 BYD Qin EV. Multiple harness measurements normal, but the DTC persisted. Checked the SRS ECU ignition pulse output with an oscilloscope—waveform abnormal. Replaced the SRS control unit (synchronized vehicle VIN and configuration codes), performed 'ECU coding' and 'crash sensor calibration', and resolved the fault. Root cause: internal ignition driver MOSFET breakdown.
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.