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B178812

This DTC indicates a short to vehicle power positive (B+) in the driver-side front airbag Stage 2 inflator circuit — Seal U

Safety System

This DTC indicates a short to vehicle power positive (B+) in the driver-side front airbag Stage 2 inflator circuit.

In modern dual-stage airbag systems, separate circuits control the Stage 1 and Stage 2 inflators, triggering them in stages based on collision severity (Stage 1 low power, Stage 2 high power).

A short to power means the airbag control unit (ACU) detects the Stage 2 inflator circuit voltage remaining abnormally high (12V).

This prevents this airbag stage from deploying correctly during a collision (because the power supply bypasses the trigger circuit) and creates a major safety hazard of unintended airbag deployment, as the short-circuit current can accidentally trigger the inflator.

The fault may originate in the clock spring, main wiring harness connector, airbag module, or the ACU.

4
Cases Logged
Real-World Cases
BYD DTC AI Analysis

2019 Qin EV clock spring internal short caused DTC B178812

✓
Airbag warning light stayed on when the vehicle arrived at the shop. Read DTC B178812. Found the code cleared with the steering wheel at full left lock, but returned when centered. Removed the steering wheel and found insulation damage between the stage 2 igniter circuit and power supply wiring inside the clock spring. Replaced the clock spring (part number: BYD-3773110). Fault resolved. Analysis: repeated steering wheel rotation over long-term use caused fatigue damage to the internal wiring.
BYD DTC AI Analysis

E2 ride-hail: Airbag false positive for short-to-power after accident repair

✓
An E2 rideshare vehicle was in a minor frontal collision. After an outside repair shop replaced the front bumper and airbag control unit (ACU), fault code B178812 set and would not clear. Inspection revealed the ACU-to-steering wheel harness retaining clip was missing where the harness passes through the firewall. The harness rubbed against the steering column bracket, wearing through the insulation until the copper core contacted the bracket (ground). The bracket had a high-resistance connection to battery positive through some path. Re-wrapping the harness and fitting a protective sleeve cleared the fault.
BYD DTC AI Analysis

E3: Corroded airbag connectors in flood-damaged vehicle

✓
2020 E3 fault code B178812 appeared one month after flood damage repairs. Found green copper corrosion inside the driver airbag connector (yellow). Electrolyte had formed a conductive path between the second stage ignition terminal and the adjacent power terminal (typically for the horn or multifunction buttons). Cleaned the connector and treated with electronic contact restorer. Insulation resistance returned to normal. Recommend replacing the waterproof airbag connector seal to prevent further water ingress.
BYD DTC AI Analysis

Steering wheel modification damaged the secondary ignition line

✓
A 2019 Qin EV owner fitted a racing steering wheel then reverted to stock, triggering DTC B178812. Inspection revealed that forcible deformation of the shorting bar on the second stage airbag igniter line during disassembly had shifted the pin within the connector into contact with the power pin. Additionally, an incorrect pinout on the aftermarket clock spring used for the modification allowed 12V to enter the airbag circuit. Replacing the clock spring with a genuine unit and repairing the pins resolved the issue.
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.