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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) — Seal U

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.

3
Cases Logged
Real-World Cases
BYD DTC AI Analysis

Worn right B-pillar wiring harness caused intermittent short circuit

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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.

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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 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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.