AU
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) — Atto 8

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 Atto 8 indicates a high-severity SRS fault where the right second-row seat belt pretensioner igniter circuit may be 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 and water-corroded connectors. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from 3 recorded cases.
3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Right B-pillar or floor wiring harness insulation wear: Long-term vibration, friction, or seat adjustment damages the insulation on the second-row seat belt pretensioner wiring harness (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 tube, or improper car washing allows water into the right middle-row seat belt pretensioner connector (usually located under the seat or at the base of the B-pillar). This causes electrolytic corrosion between the terminals, forming a conductive path that shorts the igniter terminal to the power terminal.
  • 3Pretensioner body internal short circuit: Manufacturing defects, aging, or moisture ingress cause the internal bridge wire of the seat belt retractor pretensioner igniter to short to the housing, or leaking igniter compound causes internal conductivity.
  • 4Incorrect wiring or modifications: During vehicle modifications (such as installing seat heaters, ambient lighting, or dash cams), technicians incorrectly tapped a power wire into the SRS harness, or improper securing caused the seat slide rail to crush and damage the harness.
  • 5SRS control unit internal fault: Component breakdown (e.g., MOSFETs, capacitors) in the ECU internal drive or monitoring circuits causes an internal short circuit between the squib driver terminal and the power supply (relatively uncommon 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. Never use a standard multimeter resistance setting to measure directly near the airbag system (use a dedicated airbag resistor or diagnostic tool guided function) to prevent accidentally triggering the pretensioner.
  • 2
    Fault confirmation and data recording: Use the BYD dedicated diagnostic tool (VDS or ED400) to read the complete DTC snapshot data. Record the voltage, resistance values, and environmental data at the time the fault occurred. Confirm whether B178112 is a current fault (Active) or a history 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 marks. Inspect the wiring harness corrugated 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 of the pretensioner circuit to the power supply. Normal value: >10 MΩ. Use a multimeter on the voltage setting 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 (e.g., B178111 or high resistance) to determine whether the issue is in the wiring harness or the pretensioner. If the fault remains, 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 middle-row right floor wiring harness routing (usually along the floor centerline or right door sill) and strip the corrugated conduit section by section to inspect. Focus on the seat mounting bracket edges, wiring harness retaining clips, and areas contacting the body metal. Repair damaged insulation or 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, then perform the 'SRS system self-check' or 'Output Test' to confirm B178112 does not return. Finally, perform a road test to confirm 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

✓
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 Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.