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B178112

This fault code indicates the SRS (Supplemental Restraint System) ECU detected an abnormally low-resistance connection between the right second-row seat belt pretensioner igniter circuit and the vehicle power supply (12V constant power or ignition switch power) — Qin Plus

Safety System

This fault code indicates the SRS (Supplemental Restraint System) ECU detected an abnormally low-resistance connection between the right second-row seat belt pretensioner igniter circuit and the vehicle power supply (12V constant power or ignition switch power).

Under normal conditions, the pretensioner igniter circuit maintains a high-resistance state, consisting only of the 2-3Ω igniter resistance and the wiring harness resistance, and must not have continuity with the power supply line.

A short to power causes the ECU to detect an abnormally high voltage (near battery voltage), triggering the fault code and illuminating the airbag warning lamp.

This fault presents a serious safety risk.

The pretensioner may deploy unintentionally while driving, suddenly tightening the seat belt and injuring the occupant.

Additionally, during an actual collision, circuit protection mechanisms or energy loss through the short circuit may prevent the pretensioner from deploying, failing to protect the occupant.

The B178112 fault code on the Qin Plus indicates a high-severity SRS fault where the right second-row seat belt pretensioner igniter circuit is shorted to 12V power. This can trigger the airbag warning light and may cause unintended pretensioner deployment or failure in a crash. Common causes include damaged B-pillar or floor wiring insulation, or water ingress and corrosion at the connector. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Right B-pillar or floor wiring harness insulation wear: Long-term vibration, friction, or seat adjustment damages the second-row seat belt pretensioner wiring harness insulation (usually located inside the B-pillar trim panel or under the seat), causing a short circuit to vehicle 12V power lines (such as constant power, ignition power, or lighting circuits).
  • 2Connector water ingress and corrosion: Vehicle wading, a blocked sunroof drain, or improper car washing forces water into the right middle-row seat belt pretensioner connector (usually located under the seat or at the base of the B-pillar). The resulting electrolytic corrosion between the terminals forms a conductive path, shorting the igniter tube terminal to the power supply terminal.
  • 3Internal short circuit in the pretensioner body: Manufacturing defects, aging, or moisture ingress cause a short circuit between the internal bridge wire and the housing of the pretensioner igniter inside the seat belt retractor, or leaking pyrotechnic material causes internal conduction.
  • 4Incorrect wiring or modifications: During vehicle modifications (such as adding seat heating, ambient lighting, or a dashcam), technicians incorrectly tap the power wire into the SRS wiring harness, or the seat slide rail crushes and damages an improperly secured harness.
  • 5SRS control unit internal fault: Breakdown of components (such as MOSFETs or capacitors) in the ECU internal drive or monitoring circuit causes an internal short circuit between the squib driver terminal and the power supply (relatively rare but requires inspection).
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 3 minutes to fully discharge the SRS capacitor. Wear an anti-static wrist strap. Do not use a standard multimeter resistance setting to take direct measurements near the airbag system (use a dedicated airbag resistor or the diagnostic tool guided function) to prevent accidental pretensioner deployment.
  • 2
    Fault Confirmation and Data Recording: Use the dedicated BYD diagnostic tool (VDS or ED400) to read the complete DTC snapshot data. Record the voltage, resistance values, and environmental data at the time of the fault. Confirm whether B178112 is a current fault (Active) or a historical fault (History).
  • 3
    Visual inspection: Remove the right middle-row B-pillar lower trim panel and seat. Inspect the seat belt pretensioner connector (yellow dedicated plug) for looseness, water ingress, corrosion, or burn damage. Inspect the corrugated wiring conduit for damage or crush marks.
  • 4
    Circuit insulation and continuity test: Disconnect the right middle-row pretensioner wiring harness from the SRS ECU connector. Use an insulation resistance tester (megohmmeter, 500V range) to measure the insulation resistance between the pretensioner circuit and the power supply. Normal value: >10 MΩ. Set a multimeter to the voltage range to check the wiring harness for abnormal voltage to the power supply.
  • 5
    Pretensioner unit test: Replace the pretensioner with a dedicated airbag resistor (2Ω). Observe if the fault code changes to an open-circuit fault (such as B178111 or high resistance) to determine if the issue is in the wiring harness or the pretensioner. If the fault persists, the wiring harness or ECU is faulty. If the fault changes, the pretensioner has an internal short circuit.
  • 6
    Detailed wiring harness inspection: Follow the routing of the middle-row right floor wiring harness (usually along the floor centerline or right door sill). Open the corrugated conduit section by section to inspect. Focus on the edges of the seat mounting brackets, wiring harness retaining clips, and points of contact with the vehicle body metal. Repair damaged insulation. Replace the wiring harness assembly if necessary.
  • 7
    SRS ECU verification: If the wiring harness and pretensioner are normal, measure continuity from the corresponding SRS ECU pin to power. If the ECU is internally shorted, replace the SRS control unit and perform coding and immobilizer matching.
  • 8
    Repair verification: After repairing or replacing the faulty component, reconnect all connectors and restore battery power. Use the diagnostic tool to clear the fault code. Perform the 'SRS system self-check' or 'collision output test' (Output Test) to confirm B178112 does not return. Finally, perform a road test to verify the airbag warning light is off and the system has no new fault codes.
Real-World Cases
BYD DTC AI Analysis

Worn right B-pillar wiring harness caused intermittent short circuit

✓
Vehicle: Qin EV 2019. Customer reported the airbag warning light occasionally comes on, particularly after adjusting the middle row seatback angle. VDS scan showed DTC B178112 (active fault). Removed the right middle row B-pillar trim and found the seatbelt pretensioner harness insulation worn through at the retainer clip below the B-pillar, exposing copper and contacting the constant 12V power wire for the rear door speakers. Seat adjustment pulled the harness, causing intermittent shorts. Repaired the damaged section with heat-shrink tubing, re-routed the wiring to clear moving parts, and relocated the mounting point to increase slack. Fault resolved. No recurrence at 1-month follow-up.
BYD DTC AI Analysis

Connector corrosion after water wading caused the fault

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Vehicle: BYD E2 2019. Water entered the cabin through a blocked sunroof drain tube and flooded the floor. After the workshop drained the water and dried the interior, the owner found the airbag warning light stayed on. The scan tool showed DTC B178112 and multiple seat occupancy sensor faults. Inspection revealed obvious water stains and green copper corrosion inside the right middle-row seatbelt pretensioner connector (located at the lowest point under the seat). An electrolyte conductive path had formed between the igniter terminal and the adjacent power terminal (which supplies the seat heater, optional on this vehicle). Repair: Replaced the pretensioner wiring harness connector (including the waterproof seal), cleaned the ECU-side connector, and treated the pins with dedicated contact cleaner. The pretensioner itself was undamaged, so we did not replace the retractor assembly.
BYD DTC AI Analysis

Internal short circuit in the pretensioner body caused abnormal resistance.

✓
Vehicle: BYD E3 2020. Accident-repaired vehicle (right rear side collision). Replaced the right second-row seat belt assembly (includes pretensioner). After installation, DTC B178112 appeared and would not clear. Checked new component wiring connections – normal. Measured pretensioner resistance at only 0.5Ω (normal: 2.0–3.0Ω), with 12V to body ground. New part had an internal quality defect: igniter bridge wire shorted to housing. Replaced the pretensioner assembly with a genuine OEM unit (part number must match vehicle configuration; distinguish between versions with and without force limiters). Fault cleared. Use BYD genuine parts when replacing SRS components on accident vehicles. Aftermarket parts risk resistance mismatch or short circuits.
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.