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B178E

DTC B178E indicates a short to the vehicle power supply (B+, 12V) in the front passenger frontal airbag Stage 2 squib circuit — Qin Plus

Safety System

DTC B178E indicates a short to the vehicle power supply (B+, 12V) in the front passenger frontal airbag Stage 2 squib circuit.

Modern BYD models use a dual-stage airbag design: Stage 1 provides low-power inflation for moderate collisions; Stage 2 provides high-power inflation for severe collisions.

The SRS ECU internal diagnostic circuit continuously monitors the airbag squib circuit resistance (normally 2-3 Ω) and its insulation to ground and power.

The ECU sets B178E when it detects a short to power in the Stage 2 squib circuit (resistance below the specified threshold, typically <200 Ω to power).

This fault forces the airbag circuit into a fail-safe state.

In extreme cases, the short circuit could accidentally deploy the airbag during driving (despite shorting bar protection) or prevent the designed staged inflation during a collision, risking occupant injury.

The B178E fault code on the Qin Plus indicates a high-severity short to 12V power in the front passenger frontal airbag Stage 2 squib circuit. This can affect correct airbag deployment and may create a safety risk. Common causes include a worn or pinched dashboard airbag harness or an internal SRS ECU drive circuit fault. Do not probe the airbag wiring; visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Front passenger airbag wiring harness worn or pinched: Retaining clips or metal edges cut the internal dashboard wiring harness during removal and installation, shorting the igniter wire to the permanent live wire.
  • 2SRS ECU internal drive circuit fault: Airbag control module internal ignition drive transistor (IGBT/MOSFET) breakdown shorts the second-stage control terminal to power.
  • 3Front passenger airbag connector terminal issue: The connector is behind the glovebox or on the instrument panel frame. A backed-out pin, bent terminal, or water ingress corrosion causes a short circuit to an adjacent power terminal.
  • 4Wiring error after accident repair: Incorrect dashboard wiring harness routing after collision repair causes the airbag wiring harness to entangle and short-circuit against the instrument panel lighting power wire or the air conditioning control power wire.
  • 5Airbag module internal short circuit: An internal short circuit exists in the second-stage squib inside the front passenger airbag assembly. This condition is rare and unrepairable. Replace the airbag assembly.
What To Do
  • 1
    Safe power-down and wait: Turn off the ignition switch, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS ECU internal energy storage capacitor and prevent accidental airbag deployment during repair.
  • 2
    Fault code freeze frame analysis: Use the dedicated BYD diagnostic tool (VDS2000/VDS3000) to read the DTC B178E freeze frame data. Record the vehicle speed, timestamp, and ambient temperature at the time of the fault to determine if it is an intermittent fault.
  • 3
    Visual and connector inspection: Remove the front passenger lower dashboard trim panel and inspect the SRS wiring harness for obvious wear or burn marks. Disconnect the front passenger airbag connector (usually yellow) and inspect the terminals for backed-out pins, corrosion, or foreign objects.
  • 4
    Wiring insulation measurement: Use a multimeter set to the resistance range to measure the resistance between the stage 2 ignition circuit and the power supply (B+) at the ECU end. The normal reading is infinity (OL). If the meter shows continuity, trace the wiring harness to locate the short circuit, focusing on the dashboard frame pass-through holes and harness fixing points.
  • 5
    Airbag module isolation test: Disconnect the airbag module. Connect a dedicated airbag simulator (SRS Simulator) to the wiring harness connector. Clear and reread the fault codes. If the fault code disappears, the fault is internal to the airbag module. If the fault code remains, the fault is in the wiring harness or ECU.
  • 6
    ECU-side voltage check: Reconnect the battery, leave the airbag module disconnected, and turn the ignition switch to the ON position. Use an oscilloscope or multimeter to measure the voltage to ground on the second-stage ignition circuit. The normal value is close to 0V (<1V). If the reading is 12V, this confirms a short to power. Repair the wiring harness.
  • 7
    Repair and Verification: Repair the damaged wiring harness (use heat-shrink tubing or replace the entire harness section). Ensure the wiring harness maintains sufficient clearance (>50mm) from the power cable. Reconnect all components, clear the DTC, and perform an SRS system self-test (use the diagnostic tool to execute the 'System Configuration' or 'Coding' operation). Finally, perform a vehicle collision simulation test (use the diagnostic tool's 'Actuation Test' function to verify circuit continuity only, without deploying the airbags).
Real-World Cases
BYD DTC AI Analysis

Song MAX aftermarket dashboard modification caused wiring harness chafing and short circuit

✓
A 2019 Song MAX developed a dashboard rattle after the owner installed an aftermarket center touchscreen. While removing the dashboard mounting screws, the owner overlooked the harness routing and cut the passenger airbag second-stage harness on the sharp edge of the dashboard frame. The exposed copper contacted the dashboard illumination power wire (constant power). The airbag warning light stayed on with DTC B178E. Removed the dashboard assembly and found the yellow SRS harness insulation damaged, with the two internal conductors touching the red power wire. Replaced the complete harness from the SRS ECU to the passenger airbag (Part No.: BYD-SRS-HRN-02), rerouted the wiring properly using a dedicated harness protection sleeve, and cleared the fault.
BYD DTC AI Analysis

Tang DM: Internal SRS ECU short after water wading

✓
2021 Tang DM. Vehicle waded through water exceeding the door sill height; two days later the airbag warning light illuminated. Scan tool retrieved DTCs B178E and B178D (first-level short circuit). Inspection revealed water traces beneath the front passenger carpet at the SRS ECU mounting location. Technical analysis: SRS ECU seal failed, allowing water into the internal circuit board and causing a short between the second-stage ignition drive circuit and the power bus. The ECU is a sealed, non-repairable component, so we replaced the SRS control unit (Part No.: ECU-SRS-TANG-21), thoroughly removed the accumulated water from the cabin, and dried the wiring harness connectors. Fault resolved. On water-damaged vehicles, check the SRS ECU compartment sealing carefully.
BYD DTC AI Analysis

Yuan EV airbag connector pin backed out causing short circuit

✓
2019 BYD Yuan EV. Airbag warning light illuminated the day after cabin air filter replacement at the dealership (glove box removal required). Diagnosis revealed active fault code B178E. Investigation found the passenger airbag connector (mounted on the frame behind the glove box) was strained during removal and refitting. The second stage igniter circuit terminal (typically pins 3-4) backed out of the connector housing and contacted the adjacent instrument panel illumination power terminal. Repair: Used a dedicated terminal tool to reseat the backed-out pin, refitted the connector locking tab, and confirmed the terminal fully engaged. Cleared fault codes; repeated vibration testing of the wiring harness did not reproduce the fault.
BYD DTC AI Analysis

Qin Pro wiring harness polarity reversed after accident repair

✓
2020 Qin petrol. After front collision repairs, the airbag warning light stayed on. Scanned DTCs B178E (short to power) and B1781 (circuit resistance too high). Checked repair records: the front passenger airbag had deployed in the accident. The technician replaced the passenger airbag assembly but misconnected the wiring harness. Yuan EV and Qin models use dual-stage airbags with two connectors (or one 8-pin). The technician swapped the stage 1 and stage 2 plugs, ignoring polarity, and connected the stage 2 circuit to the stage 1 power monitoring line. Fix: Reconnected the plugs per wiring diagram CIR-SRS-QIN-2020. Light out.
BYD DTC AI Analysis

Tang EV SRS ECU software false positive fault

✓
2021 BYD Tang EV. No collision or modification history. Intermittent airbag warning light. DTC B178E (intermittent). Circuit testing found no obvious shorts; wiggling the harness did not reproduce the fault. Further diagnosis revealed outdated SRS ECU software (V1.2) with a known EMI sensitivity issue. High-power onboard chargers or specific radio frequencies caused false short-to-power detection in the second-stage circuit. Fix: Upgraded SRS ECU software to V2.1 via BYD VDS online programming and checked airbag ground harness tightness. No recurrence after upgrade.
Other Qin Plus 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus overview →
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.