AU
B178B-00

This fault code indicates that the resistance of the second stage (Level 2, typically the full-power deployment mode for severe collisions) inflator squib in the front passenger airbag dual-stage deployment circuit falls below the safety threshold set by the SRS ECU (generally below 1 — Atto 8

Safety System

This fault code indicates that the resistance of the second stage (Level 2, typically the full-power deployment mode for severe collisions) inflator squib in the front passenger airbag dual-stage deployment circuit falls below the safety threshold set by the SRS ECU (generally below 1.5 Ω).

In BYD dual-stage airbag systems, the first and second stages correspond to different collision severities.

The system achieves staged deployment by controlling different firing circuits.

Low resistance indicates the ECU detects an abnormal drop in circuit impedance and identifies a short circuit fault.

The system enters a safety protection state and disables the airbag to prevent accidental deployment, rendering the airbag inoperative during a collision.

This fault directly affects collision safety protection and requires immediate repair.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Poor contact or terminal oxidation at the airbag wiring harness connector: The front passenger airbag mounts inside the instrument panel. Moisture, oxidation, or looseness easily causes abnormally low contact resistance or an intermittent short circuit in the yellow wiring harness connector (usually marked F201 or similar).
  • 2Airbag module internal short circuit: Damaged gas generator squib insulation or an internal circuit short to the housing drops measured resistance below the standard value (normally 2.0-3.0Ω). This commonly occurs after a collision if an internally damaged airbag remains unreplaced.
  • 3Wiring harness damaged and shorted to ground: Dashboard removal/installation, aftermarket equipment installation (e.g., dashcam routing), or small animals chewing the harness damages the insulation layer, creating a short circuit loop with the vehicle body metal.
  • 4SRS ECU internal detection circuit fault: Damage to the ECU internal sampling resistor or A/D conversion circuit causes the second-stage deployment circuit resistance sampling value to drift, generating a false report.
  • 5Non-standard repair procedures: Using a standard multimeter to measure the airbag connector directly (use a dedicated airbag resistance tool or ensure test current <1mA), or installing a non-genuine/refurbished airbag assembly causing a resistance mismatch.
What To Do
  • 1
    Safe power-down and discharge: Turn off the ignition switch, disconnect the negative battery terminal, and wait at least 90 seconds (to fully discharge the SRS backup power capacitor). Never connect or disconnect any airbag-related connectors while the system is powered on.
  • 2
    Visual inspection and cleaning: Remove the passenger-side lower dashboard trim panel and glove box. Locate the yellow airbag wiring harness connector (F201). Check for looseness, water ingress, terminal oxidation, or trapped foreign objects. Clean with electrical contact cleaner, then unplug and reconnect the connector. Ensure the locking tab fully engages.
  • 3
    Airbag module resistance measurement: Disconnect the airbag module connector. Use a dedicated airbag resistance tester or a digital multimeter (confirm current <1mA) to measure the resistance between the airbag terminals. The standard value is 2.0-3.0 Ω. If the resistance is <1.5 Ω or close to 0 Ω, this indicates an internal short circuit. Replace the airbag module.
  • 4
    Wiring harness continuity check: Disconnect the SRS ECU connector (located under the center console or at the front bulkhead) and measure the resistance to ground of the front passenger airbag stage 2 circuit (usually F201 to the corresponding ECU pin). Resistance must be >1MΩ. Measure the wiring harness continuity resistance. Resistance must be <1Ω; otherwise, repair the damaged section of the wiring harness.
  • 5
    Parts replacement: If the airbag module is faulty, replace the front passenger frontal airbag assembly with an OEM part (Note: Qin 100/Qin 80 and Qin EV300/450 airbag part numbers may differ; verify using the VIN). If the ECU is faulty, replace the SRS control unit.
  • 6
    System configuration and calibration: Connect the VDS2000/VDS3000 diagnostic tool and perform "Clear DTCs" → "SRS System Self-check" → "Write Configuration Parameters" (some models require entering the airbag barcode information). Confirm B178B-00 does not recur and the instrument cluster airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV300: Connector came loose after dashboard removal and refit, causing B178B-00

✓
A 2017 Qin EV300 showed a constant airbag warning light after the dealership replaced the air conditioning evaporator. VDS retrieved DTC B178B-00. The technician found the passenger airbag harness connector (yellow plug) had not fully seated during dashboard removal and installation, and terminal oxidation caused reduced contact resistance. The technician reseated the connector, treated the terminals with electronic cleaner, and cleared the DTC. The system self-test passed and the fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Qin 100 airbag module internal short after collision repair

✓
A 2017 Qin 100 sustained a minor frontal impact. External damage was slight and the airbags did not deploy, but the instrument cluster displayed an airbag fault after repairs. Diagnosis revealed DTC B178B-00; measured resistance on the passenger airbag second stage circuit was only 0.6Ω (normal 2.0–3.0Ω). Disassembly revealed the airbag inflator internal bridge wire had deformed from the impact, causing a short to the housing. Replaced the OEM passenger front airbag assembly (part number: ...verify against vehicle specification) and completed system calibration. Fault resolved.
BYD DTC AI Analysis

Qin EV450 intermittent fault from connector oxidation after water wading

✓
A 2018 Qin EV450 developed an intermittent airbag warning light after driving through deep water in heavy rain; the light sometimes cleared after a restart. A VDS scan retrieved historic fault code B178B-00. Inspection revealed damage to the passenger-side floor harness grommet, letting moisture into the airbag harness connector. This reduced insulation resistance between terminals and created a micro-short. Fix: thoroughly cleaned the connector terminals, dried the harness interior with a heat gun, applied waterproof insulating silicone grease to the connector, and installed a waterproof protective sleeve. The fault has not returned.
BYD DTC AI Analysis

Aftermarket modification pinches wiring harness, causing short on Qin 80

✓
A 2017 Qin 80 owner self-installed a Head-Up Display (HUD), routing the wiring through a dashboard gap and accidentally pinching the passenger airbag harness. After some time, the airbag warning light illuminated with DTC B178B-00. Inspection found the harness insulation chafed against the dashboard metal bracket, causing the second-stage trigger wire to short to ground. The repair involved fixing the damaged harness (replacing the entire section or wrapping locally), repositioning the HUD wiring to avoid the airbag harness, and clearing the fault codes. The system then returned to normal.
Other Atto 8 Fault Codes
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.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.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.