AU
B178C

This DTC indicates the SRS (Supplemental Restraint System) ECU detected the circuit resistance of the front passenger-side second-stage frontal airbag (a dual-stage inflator using staged ignition technology) exceeds the calibrated upper limit (normal value approximately 2 — Seal U

Safety System

This DTC indicates the SRS (Supplemental Restraint System) ECU detected the circuit resistance of the front passenger-side second-stage frontal airbag (a dual-stage inflator using staged ignition technology) exceeds the calibrated upper limit (normal value approximately 2.0-3.0Ω, fault threshold typically >3.5Ω or open circuit).

The second-stage airbag deploys in stages based on the collision severity sensor signal (delayed ignition) to reduce occupant impact during a low-speed collision.

Excessive resistance indicates a high-resistance connection or potential open circuit.

Oxidized or corroded connectors, loose wiring harness connections, fatigue fracture of the spiral resistance wire inside the airbag inflator, or an abnormal ECU sampling circuit can cause this condition.

This fault causes the airbag to deploy only in single-stage mode during a collision (delayed ignition failure) or fail completely, severely degrading crash protection performance.

This is a safety-critical fault.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Loose, oxidized, or poor contact at the front passenger airbag module wiring harness connector (common in water-damaged vehicles or after prolonged use in high-humidity environments)
  • 2Airbag wiring harness pinched and chafed where it passes through the centre console frame or near the steering column, breaking the copper wire without completely damaging the insulation (hidden high resistance).
  • 3Resistance wire inside the second-stage gas generator igniter is broken due to aging or has a poor solder joint (internal airbag assembly fault; unrepairable, replace the complete assembly).
  • 4SRS wiring harness damaged during vehicle modifications (such as installing a large center display or dash cam), or connector not fully seated.
  • 5Airbag ECU internal A/D conversion circuit or sampling resistor fault (confirm using the standard resistor bridging method).
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 3 minutes to fully discharge the SRS capacitor, then enter safe repair mode.
  • 2
    Initial inspection: Remove the passenger-side glove box or lower airbag trim panel. Visually inspect the yellow SRS connector for backed-out terminals or pin oxidation (green corrosion). Measure resistance between the connector terminals (standard: 2.0-3.0 Ω).
  • 3
    Circuit inspection: If resistance exceeds the limit, disconnect the airbag connector and bridge the airbag-side connector with a jumper wire. Measure circuit continuity (should be <1Ω) and insulation (resistance to ground >1MΩ) at the ECU end. Focus inspection on the wiring harness sleeve at the edge of the center console frame.
  • 4
    Component replacement: If the wiring is normal but the airbag-side resistance is >3.5 Ω, confirm an internal fault in the second-stage inflator and replace the front passenger airbag assembly. (Note: Do not measure airbag resistance directly using a multimeter in resistance mode; use a dedicated airbag simulator.)
  • 5
    System verification: Reconnect all connectors and the battery. Clear the fault code using the diagnostic tool. Perform the 'SRS system self-test' and verify it passes. Finally, perform a static crash simulation test (use a 2.7Ω substitute resistor to verify ECU recognition).
Real-World Cases
BYD DTC AI Analysis

Intermittent B178C fault on Song MAX after water wading

✓
A 2019 Song MAX showed an intermittent airbag warning light after driving through heavy rain and standing water. The scanner read B178C (current fault). Technicians found the passenger airbag connector under the center console had retained floodwater, causing slight pin oxidation (grey-green rust spots). Continuity remained, but contact resistance measured 4.2Ω. Cleaning the connector and applying specialized conductive paste fixed the fault. Seal SRS connectors on any vehicle that has been through flood water.
BYD DTC AI Analysis

Yuan EV second-stage airbag resistance high after accident repair

✓
2019 BYD Yuan EV. DTC B178C set three months after front collision repairs. The shop replaced the passenger airbag using an aftermarket wiring harness. Inspection found a section of secondary airbag circuit harness pinched by the sharp edge of the center console frame, causing 50% of internal copper wires to fracture and high resistance (3.8Ω). Replaced with genuine SRS harness and rerouted to avoid interference. Fault resolved. Note: Pay attention to harness routing and protection during accident repairs.
BYD DTC AI Analysis

Tang DM aftermarket large screen installation caused loose connector

✓
2021 BYD Tang DM. One week after the owner fitted an aftermarket 12.3-inch centre touchscreen, a B178C fault appeared. Inspection revealed the installer had tugged on the SRS wiring harness behind the glove box during the retrofit, causing the passenger-side second-stage airbag connector (normally concealed behind the glove box) to work loose by approximately 2 mm. While not fully disconnected, this increased contact resistance. The technician reseated the connector and secured it with an additional clip. The fault code cleared and has not returned. When removing or refitting the centre console, check the SRS connector condition.
BYD DTC AI Analysis

Airbag gas generator internal resistance wire fractured

✓
A Qin Pro EV with 80,000 km and no accident history logged DTC B178C. Wiring continuity checked normal. Direct measurement at the airbag assembly connector showed infinite resistance (open circuit). Diagnosis: fatigue fracture of the igniter resistance wire inside the second-stage gas generator—an internal airbag assembly fault. Replaced the passenger side airbag assembly with a new unit (part number must match vehicle configuration; distinguish between versions with and without side airbags). Programming and matching cleared the fault. Typical component aging case.
Other Seal U 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: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.