AU
B178F

DTC B178F indicates a short to ground in the stage 2 deployment circuit of the front passenger dual-stage frontal airbag — Seal U

Safety System

DTC B178F indicates a short to ground in the stage 2 deployment circuit of the front passenger dual-stage frontal airbag.

In a dual-stage airbag system, the stage 1 and stage 2 inflators trigger in stages based on collision severity: stage 1 provides basic protection, and stage 2 supplements inflation during a severe collision.

This fault means the SRS control module detects an abnormally low resistance (typically below 1.0 Ω) between the stage 2 inflator circuit and body ground.

Possible causes include an internal short circuit in the clock spring (spiral cable), damaged airbag wiring harness insulation contacting the metal frame, corroded connector terminals causing a ground fault, or an internal short circuit in the airbag module inflator.

This fault prevents the stage 2 airbag from deploying normally, reduces protection performance during a severe collision, forces the SRS system into fail-safe mode, and illuminates the airbag warning lamp.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1A broken internal wire or worn insulation in the clock spring (spiral cable) shorts the second-stage ignition circuit to the grounded housing. Frequent steering wheel rotation commonly causes this fatigue damage.
  • 2Vibration and chafing inside the dashboard or near the A-pillar damaged the insulation on the wiring harness between the front passenger airbag module and the SRS ECU, causing it to contact the metal body frame and short to ground.
  • 3Water enters the airbag connector (usually located under the dashboard or at the airbag module interface) due to wading, vehicle washing, or poor sealing, causing electrolytic corrosion between terminals and a short to ground.
  • 4Internal fault in the SRS control module ignition driver circuit causing a false short-to-ground indication, or a short circuit between the airbag igniter internal bridge wire and the housing (very rare, but cannot be ruled out).
  • 5Improper repair procedures, such as directly measuring the igniter circuit using a multimeter resistance setting (which can damage the internal SRS ECU circuit), or incorrectly connecting the airbag harness when installing aftermarket electronic equipment.
What To Do
  • 1
    Safety preparation: Set the vehicle to OFF, disconnect the negative terminal of the 12V low-voltage battery, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use a dedicated BYD diagnostic tool (ED400 or X431) to read the DTC. Confirm B178F is a current fault (not a history fault). Record the freeze frame data and check for accompanying fault codes (e.g., B178E, B178D).
  • 3
    Visual inspection: Remove the passenger-side dashboard trim panel. Inspect the airbag module connector (usually a yellow plug) for looseness, water ingress, or corrosion. Inspect the wiring harness near the steering column and at the dashboard frame mounting points for signs of chafing.
  • 4
    Circuit measurement: Disconnect the SRS ECU and airbag module connectors. Use a multimeter to measure the resistance to ground of the Stage 2 igniter circuit (usually the black/yellow wire). Normal resistance is infinity (OL). Continuity indicates a short to ground.
  • 5
    Section-by-section troubleshooting: If the circuit has continuity, use a clock spring tester or section-by-section measurement method to measure the insulation of the clock spring, instrument panel wiring harness, and floor wiring harness separately to locate the exact position of the short circuit.
  • 6
    Component replacement: If the clock spring is faulty, replace the clock spring (center the steering wheel during installation); if the wiring harness is damaged, repair the wiring harness and reinforce insulation protection; if suspecting an internal short circuit in the airbag module, replace the front passenger airbag assembly.
  • 7
    System verification: Reconnect all connectors, reconnect the battery negative terminal, use the diagnostic tool to clear the fault code, and perform the SRS system self-check. Confirm B178F does not return and verify the airbag warning light turns off after the self-check.
  • 8
    Functional test: Perform a static test of the crash sensors and airbag system (without deploying the airbags). Confirm the SRS ECU communicates normally with all airbag modules and sets no new fault codes.
Real-World Cases
BYD DTC AI Analysis

Clock spring internal short caused B178F

✓
A 2020 BYD Tang DM with 80,000 km displayed “Please check SRS system.” Found codes B178F and B178E (Stage 2 short to ground and short to power). Disassembly revealed the clock spring (spiral cable) beneath the steering wheel had worn insulation on pins 4 and 5 (Stage 2 igniter circuit) from constant rotation; the wires contacted the metal housing and shorted to ground. Replaced the clock spring (Part No.: KJ-3402100). Igniter resistance measured 2.3Ω. Cleared the codes and the system returned to normal.
BYD DTC AI Analysis

Dashboard wiring harness chafed and shorted to ground

✓
2019 BYD Yuan EV. Airbag warning light illuminated after wading through water. Scan tool showed active fault code B178F. Inspection found the passenger airbag harness chafed through at the right-side dashboard mounting clip due to vibration; worn insulation exposed bare copper, which contacted the metal dashboard bracket causing a short to ground. Water ingress accelerated the harness deterioration. Repair: Wrapped the damaged section with insulation tape, rerouted the harness to avoid sharp metal edges, added corrugated tubing protection, dried the connectors, and cleared the fault.
BYD DTC AI Analysis

Corroded airbag connector due to water ingress

✓
2021 BYD Qin Pro used in a high-rainfall southern region; the passenger-side A-pillar had previous panel beating repairs. The airbag warning light illuminated intermittently, with DTC B178F stored as an intermittent fault. Removed and inspected the passenger airbag connector (located behind the glovebox) and found green corrosion on the terminals with obvious water stains. Poor sealing during the A-pillar repair allowed rainwater ingress. Cleaned the connector terminals, sprayed electronic cleaner, replaced the waterproof seal, and repaired the A-pillar water barrier. The fault has not reoccurred.
BYD DTC AI Analysis

SRS ECU false positive fault

✓
2019 BYD Song MAX. Replaced the passenger airbag after accident repairs, but the airbag warning light remained on with DTC B178F. New airbag secondary igniter resistance measured 2.1Ω (normal). Wiring harness showed good insulation to ground. Suspected SRS control module damage from the accident caused abnormal detection in the internal igniter drive circuit. Replaced the SRS ECU with an identical unit (reconfigured VIN and sensor parameters), cleared the fault code, and confirmed it did not return. Confirmed internal control module failure causing false alarm.
BYD DTC AI Analysis

Wiring error in aftermarket device caused short circuit

✓
2020 BYD Tang. Owner installed a dash cam and mistakenly spliced the power feed into the passenger airbag harness. This grounded the second-stage igniter circuit through the power wire, triggering DTC B178F. Inspection found the airbag harness cut and incorrectly reconnected. Removed the unauthorized wiring and restored the original circuit using a dedicated harness repair kit (waterproof heat-shrink tubing and soldering). Educated the owner that airbag harnesses are marked yellow and must never be modified or tapped into.
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.