AU
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 — Seal U

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.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Front passenger airbag module internal second-stage igniter short circuit: Moisture ingress or aging of the bridge wire or igniter charge inside the gas generator causes an internal short circuit, resulting in an abnormally low measured resistance.
  • 2Wiring harness short to ground or power supply: Wear, crushing, or rodent damage to the internal instrument panel wiring harness causes the stage 2 igniter circuit (usually the positive or negative wire) to short to body ground or the 12V power supply line.
  • 3Connector water ingress or corrosion: The front passenger airbag connector is located on the right side of the dashboard. Vehicle wading, A/C condensate leaks, or improper cleaning can cause oxidation or electrolysis on the connector pins, creating a low-resistance path.
  • 4SRS ECU internal diagnostic circuit fault: A damaged ECU internal A/D converter or sampling resistor causes a false low resistance reading (actual resistance is normal but measured value is low).
  • 5Improper repair work: Failing to correctly secure the wiring harness during post-accident airbag module installation, causing harness pinching, or installing a non-genuine airbag module (mismatched resistance parameters).
What To Do
  • 1
    Safety preparation: Set the vehicle to OFF, disconnect the 12V battery negative terminal, wait at least 3 minutes (to fully discharge the SRS capacitor), and wear an anti-static wrist strap.
  • 2
    Fault confirmation: Connect the VDS2000/3000 diagnostic tool. Read the DTC to confirm whether B178B is an Active or History code. Check the resistance value in the freeze frame data (usually displays <0.8Ω).
  • 3
    Visual inspection: Remove the front passenger-side lower dashboard trim panel and glove box. Check the SRS wiring harness for damage or crush marks. Check the airbag module connector (yellow) for looseness, water ingress, or green corrosion.
  • 4
    Isolation measurement: Disconnect the front passenger airbag module connector (disconnect the battery first). Use a digital multimeter (resistance setting) to measure the resistance between the Stage 2 inflator pins on the airbag side. Standard value: 2.0 ± 0.3 Ω. If <1.0 Ω, replace the airbag module.
  • 5
    Harness continuity test: Measure the resistance to ground of the corresponding pin on the ECU-side harness connector (should be >1 MΩ). Measure the resistance between the positive and negative wires of the second-stage harness (should be >1 MΩ). If abnormal, repair the harness (use twisted-pair wire and match wire colours).
  • 6
    ECU diagnosis: If the wiring harness and airbag module resistances are normal, measure the voltage at the corresponding pin of the SRS ECU connector (static voltage <1V). If the fault persists, replace and program the SRS ECU.
  • 7
    System reset: After repair, reconnect all connectors (verify the locking click). Connect the battery. Use the diagnostic tool to clear the DTC and perform the 'SRS System Check'. Confirm B178B changes to 'Passed' status.
  • 8
    Verification test: Perform a vehicle collision simulation test (using a dedicated resistor to simulate the load, not a real airbag) or a road test. Confirm the SRS warning lamp turns off after the self-check and does not illuminate again.
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 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.