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 3

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 3
  • 1Poor contact or oxidized terminals at the airbag wiring harness connector: The front passenger airbag mounts inside the dashboard. Moisture, oxidation, or a loose connection often causes the yellow wiring harness connector (usually marked F201 or similar) to exhibit abnormally low contact resistance or an intermittent short circuit.
  • 2Airbag module internal short circuit: Damaged gas generator squib insulation or an internal circuit-to-housing short drops the measured resistance below the standard value (normally 2.0-3.0Ω). This commonly occurs when an internally damaged airbag remains unreplaced after a collision.
  • 3Wiring harness damaged and shorted to ground: Dashboard removal/installation, aftermarket equipment installation (such as dash cam wiring), or animal chewing damages the harness insulation, creating a short circuit to the vehicle body metal.
  • 4SRS ECU internal detection circuit fault: A damaged ECU internal sampling resistor or A/D conversion circuit causes the resistance sampling value of the second-stage deployment circuit to drift, resulting in a false fault report.
  • 5Non-standard repair procedures: Measuring the airbag connector directly with a standard multimeter (use a dedicated airbag resistance tool or limit test current to <1mA), or installing a non-OEM/refurbished airbag assembly, causing a resistance mismatch.
What To Do
  • 1
    Safe power-off 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 with the power 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 disconnect and reconnect the connector, ensuring the locking tab engages fully.
  • 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, the airbag module has an internal short circuit. Replace the airbag module.
  • 4
    Harness continuity check: Disconnect the SRS ECU connector (located under the center console or bulkhead). Measure the resistance to ground of the front passenger airbag Stage 2 circuit (usually F201 to the corresponding ECU pin); resistance must be >1 MΩ. Measure the harness continuity resistance; resistance must be <1 Ω. Otherwise, repair the harness damage.
  • 5
    Parts replacement: If the airbag module is faulty, replace the OEM front passenger frontal airbag assembly (Note: Airbag part numbers for Qin 100/Qin 80 and Qin EV300/450 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 return 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 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.