AU
B178A-00

DTC B178A-00 indicates the SRS (airbag) electronic control unit detects an abnormally low resistance (close to 0 ohms) in the front passenger Stage 2 frontal airbag ignition circuit, indicating a short circuit (short to ground or short between wires) — Seal U

Safety System

DTC B178A-00 indicates the SRS (airbag) electronic control unit detects an abnormally low resistance (close to 0 ohms) in the front passenger Stage 2 frontal airbag ignition circuit, indicating a short circuit (short to ground or short between wires).

In a dual-stage ignition system, the Stage 2 airbag provides additional protection during severe collisions, typically deploying simultaneously with Stage 1 or triggering with a delay.

A resistance of 0 ohms causes the SRS ECU to determine the ignition circuit is unreliable, illuminate the airbag warning light, and disable the front passenger airbag Stage 2 deployment function.

In extreme cases, the airbag may deploy inadvertently without a collision or fail completely.

This is a hard fault.

The ECU stores it continuously until repaired.

The B178A-00 fault code on the Seal U indicates a low-resistance short in the front passenger Stage 2 frontal airbag ignition circuit. Severity is medium because it can disable Stage 2 airbag deployment, turn on the airbag warning light, and in rare cases cause unintended deployment. Common causes include a shorted passenger airbag igniter or damaged wiring harness insulation. The recommended fix is to visit an authorised dealer for SRS diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Front passenger airbag module internal second-stage igniter short circuit (damaged internal coil insulation causes short circuit between positive and negative terminals)
  • 2Airbag wiring harness short to body ground (harness wear, crushing, or aging damages insulation, allowing contact with the metal vehicle frame)
  • 3Water ingress or corrosion in the front passenger side airbag connector (A/C condensate leak or driving through water causing a short circuit between connector pins)
  • 4SRS control unit internal monitoring circuit fault (damaged ECU internal sampling resistor or comparator circuit causing false reporting)
  • 5Improper repair procedures (using a standard multimeter to measure airbag pins directly, causing an internal short circuit, or damaging the wiring harness with tools during removal and installation)
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (some models require 3 minutes) to completely discharge the SRS capacitor and prevent accidental deployment.
  • 2
    Fault confirmation: Read and record the fault code using a BYD dedicated diagnostic tool (VDS or ED400). Confirm B178A-00 is a current fault (Active). Check the environmental conditions recorded in the freeze frame data.
  • 3
    Visual inspection: Check the front passenger airbag module, the wiring harness under the instrument panel, and the floor wiring harness connector (usually on the right side of the centre console or behind the glove box) for obvious damage, water stains, or burn marks.
  • 4
    Resistance measurement: Disconnect the airbag connector. Use a dedicated airbag tester (or a high-impedance digital multimeter) to measure the resistance between the Stage 2 igniter pins. The normal value is 1.5-3.0 Ω. If the resistance is 0-0.5 Ω, replace the airbag module.
  • 5
    Harness continuity test: Measure the resistance to ground of the second-stage circuit at the harness-side connector (should be >10 kΩ or infinite). Measure wire-to-wire insulation (resistance between positive and negative wires should be infinite). Focus inspection on the harness fixing points routed through the center console.
  • 6
    Simulation verification: Replace the actual airbag with a 2Ω airbag simulator, clear the fault code, turn on the power supply, and perform an SRS self-check. If the fault code disappears, the wiring harness and ECU are normal, and the fault lies in the airbag module.
  • 7
    Component replacement and verification: Based on diagnostic results, replace the front passenger airbag module (use new clips and guide sleeves), repair the wiring harness, or replace the SRS ECU. After reassembly, perform 'SRS System Configuration' and 'Crash Sensor Calibration' (if applicable). Finally, perform a vehicle collision simulation test (use the diagnostic tool to trigger the test function).
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV300 front passenger airbag second-stage circuit short after water fording

✓
A 2017 Qin EV300 showed a steady SRS warning light after wading through heavy flood water. Retrieved DTC B178A-00 (current fault). Stripped and inspected the front passenger floor wiring harness; the waterproof grommet sealed poorly. Water ingress oxidised and shorted the airbag connector pins. Repair: Thoroughly dried the harness, cleaned the connector with electronic contact cleaner, applied conductive anti-corrosion grease, and replaced the damaged airbag wiring harness branch. After reassembly, cleared the DTCs and the SRS system self-test passed. Fault resolved.
BYD DTC AI Analysis

Qin 100 dashboard wiring harness pinched and shorted after accident repair

✓
After the accident-damaged vehicle had its front bumper, dashboard and passenger airbag replaced, it immediately set DTC B178A-00 on startup. Inspection revealed the technician failed to route the passenger side second-stage airbag harness along its original path when installing the dashboard. The harness became pinched against the metal edge of the dashboard frame, chafing through the insulation and causing the positive and negative conductors to short together. Repair: Removed the damaged section of wiring, soldered the connections and insulated with heat-shrink tubing. Re-routed and secured the harness according to factory specifications, keeping it away from sharp edges and high-temperature components. Fault resolved.
BYD DTC AI Analysis

Manufacturing defect in the airbag module internal igniter caused zero resistance.

✓
A 2018 BYD Qin EV450's SRS warning light illuminated suddenly at 20,000 km without collision. The diagnostic scanner showed static fault code B178A-00. Disconnecting the airbag connector revealed 0.3Ω resistance at the module-side second-stage pins (normal ~2.0Ω). This indicated an internal short caused by a powder pressing defect in the second-stage igniter. Replaced the passenger airbag assembly (including second-stage inflator) with a new unit, reinstalled using special tools with torque set to 9-10 N·m, then programmed and matched the module. Fault cleared.
BYD DTC AI Analysis

False positive fault in SRS ECU internal sampling circuit

✓
Intermittent DTC B178A-00; normal when cold, present when hot. Measured airbag module and wiring harness resistance, both normal at 1.8Ω; harness insulation good. Suspected poor thermal stability of internal A/D converter circuit or sampling resistor in SRS ECU. Replaced airbag control unit (located beneath centre console), performed ECU coding (entered vehicle VIN and airbag configuration code) and system initialisation learning. Road test confirmed fault did not recur.
BYD DTC AI Analysis

Bent connector pin caused positive and negative terminals to contact

✓
The fault appeared after routine maintenance at the dealership. Root cause: the technician bumped the passenger airbag connector while removing the glovebox to check the cabin filter, bending the second-stage circuit pins until they contacted and shorted. Fix: Disconnected the battery, carefully straightened the pins, confirmed no fractures, reseated the connector, applied contact protectant, and cleared the codes. Protect SRS connectors during service.
Other Seal U Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U overview →
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.