AU
B178E-00

This DTC indicates the airbag control unit (SRS ECU) detects a short to vehicle power (12V/B+) in the front passenger airbag Stage 2 inflator circuit — Seal U

Safety System

This DTC indicates the airbag control unit (SRS ECU) detects a short to vehicle power (12V/B+) in the front passenger airbag Stage 2 inflator circuit.

Dual-stage airbags feature two independent inflators for different crash severities: Stage 1 provides cushioning in minor collisions, and Stage 2 provides additional protection in severe collisions.

A short to power indicates an abnormal electrical connection between the inflator wiring and the battery positive terminal.

This fault causes the SRS system to enter fail-safe mode, illuminates the airbag warning light, and disables the front passenger airbag Stage 2 deployment function.

In extreme cases, it causes unintended airbag deployment or prevents normal deployment during a collision, creating a severe safety risk.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Damaged internal wire insulation in the airbag clock spring (spiral cable/coil spring) shorts the Stage 2 squib circuit to the power supply line.
  • 2Damaged wiring harness insulation between the front passenger airbag module and the SRS ECU, contacting the vehicle body metal frame or power supply line.
  • 3A fixing screw crushed or chafed the instrument panel internal wiring harness during removal or installation, causing a power short circuit.
  • 4Internal failure of the SRS ECU driver integrated circuit (ASIC) causing the igniter output terminal to short internally to power.
  • 5Water ingress, corrosion, or bent pins in the front passenger airbag connector (yellow connector) causing a short circuit between the power terminal and squib terminal.
What To Do
  • 1
    Safety preparation: Turn off the vehicle, disconnect the 12V battery negative terminal, and wait at least 90 seconds to completely discharge the SRS backup power supply and prevent accidental airbag deployment.
  • 2
    Fault Confirmation: Connect the VDS or dedicated diagnostic tool. Read the complete fault codes and freeze frame data. Verify B178E-00 is a current fault (not a history fault). Record the environmental data from when the fault occurred.
  • 3
    Visual inspection: Check the front passenger airbag area, instrument panel, and centre console for signs of impact, modification, or non-genuine equipment installation. Specifically inspect the yellow SRS wiring harness for damage, pinching, or burn marks.
  • 4
    Connector inspection: Disconnect the front passenger airbag module connector (usually located behind the airbag or under the dashboard) and the clock spring connector. Inspect the terminals for corrosion, water ingress, burns, or abnormal pin contact.
  • 5
    Circuit continuity test: Use a multimeter to measure the resistance from the second-stage squib terminal on the airbag wiring harness side to body ground and to the power supply. The normal reading is an open circuit (infinite resistance). Continuity or an abnormal resistance value indicates a short circuit.
  • 6
    Clock spring test: Measure the continuity of the second-stage squib circuit between the inner and outer rings of the clock spring (spiral cable). Turn the steering wheel to the left and right lock positions. If the resistance changes or a short circuit occurs, replace the clock spring.
  • 7
    Insulation resistance measurement: Use a megohmmeter to measure the insulation resistance between the Stage 2 squib circuit and the power supply. The standard value must be greater than 10MΩ. If the value is lower, inspect the wiring harness for damage.
  • 8
    Component replacement: If the wiring harness is normal, install a known-good airbag module for testing. If the fault clears, replace the airbag module. If the fault remains, inspect or replace the SRS ECU.
  • 9
    System reset: After completing the repair, reconnect all connectors and restore 12V battery power. Use the diagnostic tool to clear the fault code and perform an SRS system self-check. Verify DTC B178E-00 does not return and the warning light turns off.
  • 10
    Function check: Read the crash sensor and airbag circuit resistance values. Verify the second-stage squib circuit resistance is within the specified range (typically 1.5-3.0Ω) with no short or open circuits.
Real-World Cases
BYD DTC AI Analysis

Clock spring internal short caused intermittent fault

✓
A 2018 BYD Qin EV450 intermittently illuminated the airbag warning light on the instrument cluster. DTC B178E-00 was retrieved. The fault appeared randomly, increasing in frequency when turning the steering wheel. The technician disconnected the airbag module and measured the harness from the clock spring to the SRS ECU, finding a short to power. Disassembling the clock spring revealed that at full steering lock, the second-stage igniter line in the internal flat cable was contacting the power supply line. After replacing the clock spring, the technician tested steering at full lock left and right; no recurrence.
BYD DTC AI Analysis

Dashboard modification pinched wiring harness, causing short circuit

✓
2017 BYD Qin 100. Owner reported constant airbag warning light, DTC B178E-00. Vehicle recently had a large centre console screen installed at an aftermarket shop. Technician found passenger-side dashboard bracket mounting screws were too long, piercing the SRS wiring harness sheath and causing a short between the second-stage igniter power and signal lines. Repaired harness damage (rewrapped with insulation), replaced damaged harness section, adjusted screw length. Cleared the fault. Advised owner to use professional repair shops for dashboard disassembly and reassembly.
BYD DTC AI Analysis

Connector corrosion and short circuit after water ingress

✓
A Qin EV300's airbag warning light illuminated after driving through deep flood water. Code B178E-00 stored and would not clear. Checked the passenger airbag connector (behind the glovebox) and found water staining and copper corrosion inside; degraded insulation between the terminals caused a short to power. Cleaned the connector with electronic contact cleaner, dried it with compressed air, and applied conductive anti-corrosion grease. Insulation resistance returned to normal. Cleared the code and the system returned to normal. Recommend checking the vehicle's water ingress sealing.
BYD DTC AI Analysis

SRS ECU internal driver chip failure

✓
2017 BYD Qin 80, no accident history. SRS warning light suddenly illuminated. Scan tool retrieved DTC B178E-00. Measured passenger airbag second stage igniter circuit to ECU — no external short. Disconnected ECU connector and measured harness side — normal. Reconnected ECU and fault returned. Disassembled SRS ECU and found internal igniter driver ASIC chip burnt out, causing internal short to power at second stage igniter output. Replaced SRS ECU, programmed and matched — fault resolved. Old ECU failed due to internal component ageing.
BYD DTC AI Analysis

Internal squib short circuit in airbag module

✓
During routine maintenance at a 4S dealership, a technician noticed the airbag warning light on in a Qin EV450 and retrieved DTC B178E-00. Measuring at the passenger airbag module connector revealed the second stage squib was shorted between both pins and to power. They disconnected the module and checked the squib pins on the module side: 0.5Ω with continuity to the housing, confirming an internal short in the second stage squib to the housing (ground/power reference). They replaced the passenger airbag assembly (including the module). The new module measured 2.1Ω and showed no continuity to the housing. Installation cleared the fault.
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: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.