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B178C-00

B178C-00 indicates the resistance of the front passenger-side Stage 2 frontal airbag ignition circuit falls outside the normal range set by the SRS ECU (typically 1 — Atto 8

Safety System

B178C-00 indicates the resistance of the front passenger-side Stage 2 frontal airbag ignition circuit falls outside the normal range set by the SRS ECU (typically 1.5–3.0 Ω).

In the BYD dual-stage airbag system, the Stage 2 gas generator provides higher deployment power during a severe collision.

High resistance indicates excessive impedance or a potential open circuit in the ignition circuit.

Potential causes include an internal open circuit in the airbag squib, poor contact at the wiring harness connector, or a clock spring fault.

This safety system hard fault forces the SRS ECU into fail-safe mode, disabling deployment of this airbag stage and severely reducing front passenger protection during a collision.

Repair immediately.

4
Cases Logged
Real-World Cases
BYD DTC AI Analysis

2018 Qin EV450: Front passenger airbag connector oxidized, causing high resistance

✓
After driving through water, the instrument panel airbag warning light remained on constantly. DTC B178C-00 stored. The technician removed the passenger glove box and found visible water stains and oxidation inside the yellow airbag harness connector. They cleaned the terminals with precision electronic cleaner, applied conductive protectant, and reconnected the connector. Resistance returned to 2.1Ω. After clearing the fault codes, the system returned to normal. Inspect the A/C drain hose and windshield sealing to prevent further water ingress.
BYD DTC AI Analysis

2017 Qin 100 — Clock spring internal open circuit caused second-stage airbag fault

✓
Customer reported the airbag warning light occasionally comes on when turning. Diagnosis found intermittent code B178C-00. Inspected the passenger airbag wiring harness and found it normal, but noted this model's passenger airbag circuit passes through a connector under the center console (functionally equivalent to a clock spring). Disassembly showed internal contact spring fatigue causing unstable contact resistance (normal when cold, increasing when hot or under vibration). Replaced the instrument panel wiring harness assembly and cleared the fault.
BYD DTC AI Analysis

BYD Qin EV300 2017 — Airbag module internal igniter degradation

✓
Purchased used vehicle with airbag warning light on; DTC B178C-00 present. Harness side connector resistance measured normal (<1Ω), but airbag module side second stage igniter resistance was 5.8Ω (severely high). Vehicle had been parked long-term in hot, humid conditions, causing the internal igniter to oxidize from moisture. Replaced genuine passenger side airbag module (part number matched to vehicle VIN), performed VCU and SRS system matching and programming. Fault cleared.
BYD DTC AI Analysis

2017 Qin 80: Connector not locked after accident repair

✓
After front-end collision repairs, the airbag warning light was on at delivery. Read DTC B178C-00. Inspection found the passenger airbag had been removed during accident repairs. The wiring harness connector wasn't fully seated on reinstallation—the CPA retainer didn't click, causing poor pin contact. Reconnected the connector firmly, confirmed it locked, cleared the DTC and verified it didn't return. Always perform a comprehensive SRS self-check after accident repairs.
Other Atto 8 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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.