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B178B

This DTC indicates the SRS (Supplemental Restraint System) ECU detects the circuit resistance of the front passenger dual-stage frontal airbag Stage 2 inflator is below the calibrated threshold (typically below 1 — Atto 3

Safety System

This DTC indicates the SRS (Supplemental Restraint System) ECU detects the circuit resistance of the front passenger dual-stage frontal airbag Stage 2 inflator is below the calibrated threshold (typically below 1.0Ω; normal range: 1.5-3.0Ω).

In a dual-stage airbag system, the Stage 2 inflator provides additional gas output during a severe collision.

Low resistance usually indicates a short circuit (short to ground, wire-to-wire short, or internal inflator short).

This condition may prevent proper airbag deployment during a collision (insufficient deployment force) or, in extreme cases, cause unintended deployment due to abnormal current.

The ECU continuously monitors the inflator circuit resistance via its internal diagnostic circuit.

If the resistance remains below the lower limit for a specified duration (typically several hundred milliseconds), the ECU sets this DTC, illuminates the SRS warning lamp, and may disable the front passenger airbag function.

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Cases Logged
Real-World Cases
BYD DTC AI Analysis

Flood damage corroded the passenger airbag connector, causing DTC B178B

✓
A 2020 BYD Qin Pro DM developed a constant SRS warning light after wading through water approximately 40 cm deep. The scan tool retrieved current code B178B, with freeze frame data showing 0.3 Ω resistance. Disassembly revealed the passenger airbag connector (located to the right of the air conditioning evaporator) had obvious water staining and copper corrosion, with insulation resistance between pins down to 0.2 Ω. Repair: Cleaned the connector with electronic contact cleaner, dried it, and applied conductive protective grease. Wiring harness and airbag module resistance measured normal at 2.1 Ω. Cleared fault codes; multiple road tests showed no recurrence. Fault resolved.
BYD DTC AI Analysis

Passenger side secondary airbag wiring harness pinched and shorted after accident repairs

✓
2021 BYD Tang. Following a front-end collision, a non-authorized workshop replaced the passenger airbag assembly. Three days after delivery, the SRS warning light illuminated with DTC B178B. Inspection revealed the technician failed to route the second-stage igniter harness (blue plug) into the dedicated wiring channel during dashboard installation, pinching the harness against the frame. Damaged insulation caused a short to the metal frame (0.5Ω to ground). Repaired by replacing the damaged harness section with waterproof heat-shrink tubing and re-routing the wiring correctly. Fault cleared.
BYD DTC AI Analysis

Airbag module internal second-stage igniter degraded and short-circuited

✓
2019 BYD Yuan EV, 60,000 km, no accident history. Intermittent SRS warning light. Scan tool showed B178B intermittent fault. Checked wiring harness and connectors – all normal. Measured airbag module second-stage igniter resistance fluctuating between 0.8–1.2Ω (standard 2.0Ω). Found internal bridgewire oxidation in the gas generator causing micro-shorting. Replaced genuine passenger-side airbag assembly (part number ...). Programming and configuration completed; fault fully resolved.
BYD DTC AI Analysis

SRS ECU sampling circuit fault: false low resistance reading

✓
2020 Song MAX. After repeatedly replacing the passenger airbag and wiring harness, DTC B178B persisted. Used an oscilloscope to measure the diagnostic pulse from the ECU to the second-stage igniter and found the internal sampling resistor in the ECU had failed, causing the calculated resistance to read consistently low. Disconnected the airbag module and connected a 2.0Ω standard resistor as a simulated load at the harness end; the ECU still reported 0.4Ω, confirming an ECU fault. Replaced the SRS control module (located below the centre console), performed online matching and coding (VIN programming), and cleared the fault.
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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.