AU
B178911

This DTC indicates an abnormally low-resistance connection (short to ground) between the driver-side dual-stage airbag Stage 2 ignition circuit and body ground — Seal U

Safety System

This DTC indicates an abnormally low-resistance connection (short to ground) between the driver-side dual-stage airbag Stage 2 ignition circuit and body ground.

In modern SRS systems, the driver airbag typically features a dual-stage inflator: Stage 1 deploys during minor collisions, and Stage 2 delays deployment during severe collisions to enhance protection.

A short to ground indicates the ECU detects an abnormally low voltage on the Stage 2 ignition circuit (typically <1Ω resistance).

Damaged wiring harness insulation, an internal clock spring short circuit, or an internal airbag module fault can cause this condition.

This fault causes the following: 1) The airbag system enters fail-safe mode; the Stage 2 airbag may fail to deploy or deploy unintentionally during a collision. 2) The SRS warning lamp remains illuminated, degrading the overall vehicle safety rating. 3) In extreme cases, short-circuit current can trigger accidental airbag deployment, posing a severe safety hazard.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal open or short circuit in the clock spring (spiral cable): Frequent steering wheel rotation causes wear and breakage of the flat cable inside the clock spring. The exposed conductor contacts the metal steering column, creating a short to ground. This is a common SRS fault location on the BYD E2/E3/Qin EV series.
  • 2Airbag wiring harness insulation damaged: Friction, crushing, or animal chewing damages the driver airbag wiring harness insulation where it passes near the steering column, causing the wire to contact metal body parts.
  • 3Airbag assembly (gas generator) internal short circuit: An internal bridge wire short in the second-stage igniter or moisture-induced conductive propellant causes the resistance between the module terminals to drop abnormally to nearly 0Ω.
  • 4Connector water ingress and corrosion: Poor sealing of the airbag connector under the steering wheel allows water ingress during wading or car washing, causing electrolytic corrosion between the pins and creating a short-to-ground path.
  • 5SRS ECU internal sampling circuit fault: A damaged internal A/D converter or comparator circuit in the control unit incorrectly identifies a normal resistance value as a short circuit condition (rare, but rule this out).
What To Do
  • 1
    Safety preparation: Turn off the ignition, disconnect the low-voltage battery negative terminal, wait at least 90 seconds (to fully discharge the SRS backup capacitor), and wear an anti-static wrist strap.
  • 2
    Fault Confirmation: Connect the diagnostic tool and read DTCs. Confirm B178911 is a current code (Active), not a history code. Record freeze frame data (crash sensor status, voltage values, etc.).
  • 3
    Visual inspection: Remove the trim panels on both sides of the steering wheel. Check the driver airbag connector (yellow) for looseness, water ingress, or corrosion. Check the clock spring connector pins for deformation or oxidation.
  • 4
    Circuit isolation test: Disconnect the battery negative terminal and unplug the driver airbag module connector. Use a multimeter to measure the resistance between the two pins on the airbag-side connector (normal: 2.1–2.8 Ω). Measure the resistance from each pin to body ground (normal: >10 MΩ; close to 0 Ω if short-circuited).
  • 5
    Clock spring test: Disconnect the clock spring from the body wiring harness. Measure continuity from the steering wheel side (airbag side) of the clock spring to the body side (ECU side). Turn the steering wheel left and right to check for intermittent short or open circuits.
  • 6
    Harness insulation check: Inspect the harness routing from the steering wheel to the SRS ECU. Specifically check the harness sleeve inside the steering column shroud for damage. Use a megohmmeter to measure the harness insulation resistance to ground.
  • 7
    Component replacement verification: If the wiring harness is normal, perform a substitution test using a known-good airbag module (Note: Never use a multimeter to directly measure the new airbag resistance). If the fault code clears, replace the airbag assembly.
  • 8
    ECU inspection: If the above checks are normal, measure the resistance to ground at the corresponding pin of the SRS ECU connector. If a short circuit exists, repair the wiring harness; otherwise, replace the SRS ECU control unit.
  • 9
    System reset and verification: Reconnect all components, clear fault codes, and perform an ignition cycle test (ON for 3 seconds, then OFF). Confirm the SRS light turns off after the self-check. Perform a road test and re-read DTCs to confirm no new faults occur.
Real-World Cases
BYD DTC AI Analysis

Internal short in BYD E2 clock spring causing B178911

✓
A 2019 BYD E2 with 32,000 km came in with the SRS warning light permanently illuminated. Retrieved DTCs B178911 and B178A11 (passenger side stage 2 short to ground). Disassembling the clock spring revealed the internal ribbon cable fractured at the 3 o'clock position; broken copper strands contacted the metal housing and shorted to ground. Replaced the clock spring assembly (P/N: BYD-SRS-CK-2019) to resolve. Root cause: the driver frequently held the steering wheel at full lock, exceeding the clock spring's rotation limit and over-stretching the cable until it fractured. Advised avoiding extended periods with the steering wheel at full lock.
BYD DTC AI Analysis

Qin EV Steering Column Wiring Harness Chafing and Short Circuit Repair

✓
2020 Qin EV. SRS warning light illuminated after the vehicle was driven through water. The scan tool showed DTC B178911. Inspection revealed the driver's side airbag harness had chafed where it passes through the steering column mounting bracket; the retaining clip had worked loose, allowing the harness to rub against the metal bracket edge until the insulation wore through and exposed the copper conductors. Water ingress provided a conductive path, causing a short to earth on the second stage airbag circuit. Re-wrapped the damaged harness using double-layer heat shrink tubing and insulation tape, adjusted the harness routing, replaced the retaining clip, and fitted a rubber grommet at the wiring aperture to prevent future chafing. Cleared the fault codes and the system returned to normal.
BYD DTC AI Analysis

E3 Airbag Module Internal Squib Short Circuit Replacement Case

✓
2020 BYD E3: SRS light on with DTC B178911 after accident repairs (front bumper and left fender replaced). Checked wiring harness and clock spring — both normal. Measured original driver's airbag module second-stage igniter pin-to-ground resistance: 0.3Ω (should be >2Ω). Diagnosed internal short in airbag gas generator. Replaced genuine driver's airbag module (including gas generator) and performed airbag system configuration write using VDS2000 programming. Fault cleared. Analysis: during the accident, the airbag controller sent a deployment signal but the airbag did not fully deploy, causing a partial short in the igniter.
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.