AU
B178D

DTC B178D indicates the front passenger second-stage frontal airbag (Passenger Frontal Airbag Stage 2) ignition circuit is disconnected or has abnormal resistance — Seal U

Safety System

DTC B178D indicates the front passenger second-stage frontal airbag (Passenger Frontal Airbag Stage 2) ignition circuit is disconnected or has abnormal resistance.

In BYD e-Platform 3.0 models featuring a distributed electronic architecture (e.g., Dolphin, Seal, Yuan PLUS), this DTC may also indicate a communication interruption between the Right Body Domain Controller (R-DCU) and the SRS module, or a sub-node fault.

The airbag utilizes a dual-stage ignition design to deploy in stages based on collision severity (stage 1 low-pressure deployment; stage 2 high-pressure supplementary deployment).

This fault prevents the front passenger airbag from executing the second-stage deployment during a collision or causes complete airbag failure, severely compromising occupant protection.

Inspect the airbag module wiring harness, connectors, clock spring (if applicable), and domain controller communication status.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Front passenger airbag stage 2 ignition circuit open or resistance exceeds threshold (>4.5Ω), preventing the SRS control unit from detecting a valid load.
  • 2Loose airbag wiring harness connector, oxidation from water ingress, or backed-out terminal pins (focus inspection on the airbag connector inside the instrument panel and the domain controller connector).
  • 3Open circuit or poor contact inside the clock spring (for models equipped with a conventional clock spring design)
  • 4Right Body Domain Controller (R-DCU) internal fault, software malfunction, or abnormal power supply causing loss of communication with the SRS subsystem (for vehicles with domain control architecture)
  • 5Internal igniter fault or internal open circuit in the SRS control unit or airbag module.
What To Do
  • 1
    Read the DTC freeze frame data using the VDS diagnostic tool, confirm the fault status as current (Active) or historical (History), and check for other related fault codes (such as B178C, U0187).
  • 2
    Disconnect the battery negative terminal and wait at least 3 minutes to ensure complete system discharge. Check the front passenger airbag module connector (located inside the right side of the instrument panel or at the right domain controller interface) for looseness or oxidation. Measure the Stage 2 ignition circuit resistance (standard value: 2.0-3.0 Ω).
  • 3
    Check the SRS control unit (or the SRS module integrated in the right domain controller) power supply (constant B+, IG power), ground, and CAN line voltages (CAN-H: 2.5-3.5V, CAN-L: 1.5-2.5V, static differential voltage approx. 0V).
  • 4
    If the vehicle uses a domain control architecture (such as e-Platform 3.0 models), check the communication status between the right body domain controller and the SRS module. Measure the terminal resistance of the CAN lines at diagnostic connector pins 6 and 14 (approximately 60 Ω) and verify if the right domain controller requires a software update.
  • 5
    Check the airbag wiring harness for continuity. Inspect the instrument panel wiring harness specifically for wear, pinching, and poor connector contact. Repair the wiring harness or replace the airbag module if necessary.
  • 6
    After replacing the airbag module or domain controller, write the SRS system configuration, calibrate the safety system, and test the airbag deployment (using dedicated simulation equipment, not a live deployment test).
Real-World Cases
BYD DTC AI Analysis

BYD Song PLUS DM-i Blind Spot Monitoring Restricted (Right Domain Controller Fault)

✓
Symptom description: Dashboard displayed "Blind Spot Function Limited" warning; blind spot monitoring switch not shown on centre touchscreen; VDS failed to detect right blind spot monitoring module. Diagnostic process: 1. Scanned with VDS; right blind spot monitoring module had no communication. 2. Inspected right body controller (right domain) and confirmed the module communicates via this unit. 3. Checked power supply, ground and CAN wiring to right domain controller; found internal fault in the controller was preventing the blind spot module from working. 4. Cross-check: Swapped right body controller and fault disappeared. Resolution: Replaced right body domain controller (right domain), reprogrammed and cleared the fault.
Original source ↗
BYD DTC AI Analysis

BYD Song Plus DM-i Multiple Switch Group Failure (Right Domain Communication Fault)

✓
Symptoms: Light switch would not turn on lights, wiper switch would not control wipers, and all steering wheel buttons did not respond. Instrument cluster displayed "Lane Departure Function Limited" and "Lane Keeping Function Limited". Diagnosis: 1. VDS scan found communication errors between left and right domain controllers. 2. Multiple switch signals route through the body domain controller; identified as inter-domain controller communication failure. 3. Checked power supply and network wiring to domain controllers; measured abnormal voltage on CAN lines. 4. Confirmed right body domain controller had corrupted internal software, blocking switch signal transmission. Resolution: Updated software on right body domain controller. Fault cleared after update. Some vehicles require replacement of the right domain controller.
Original source ↗
BYD DTC AI Analysis

BYD e2 AC not cooling (Integrated Body Controller/Right Domain fault)

✓
Symptom: AC stops cooling after running for a while. No cold air even with temperature set to minimum. Diagnosis: 1. VDS scan showed Integrated Body Controller logged fault code "AC line in high pressure or low pressure state". 2. AC pressure sensor and wiring checked normal. 3. Found Integrated Body Controller (AC controller integrated in right domain) outputting abnormal control signal to electronic expansion valve. 4. Data stream showed abnormal duty cycle from right domain controller driving expansion valve. Repair: Replaced Integrated Body Controller (right domain). Recalibrated AC system. Fault resolved.
Original source ↗
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.