AU
B178B-00

This fault code indicates that the resistance of the second stage (Level 2, typically the full-power deployment mode for severe collisions) inflator squib in the front passenger airbag dual-stage deployment circuit falls below the safety threshold set by the SRS ECU (generally below 1 — Seal U

Safety System

This fault code indicates that the resistance of the second stage (Level 2, typically the full-power deployment mode for severe collisions) inflator squib in the front passenger airbag dual-stage deployment circuit falls below the safety threshold set by the SRS ECU (generally below 1.5 Ω).

In BYD dual-stage airbag systems, the first and second stages correspond to different collision severities.

The system achieves staged deployment by controlling different firing circuits.

Low resistance indicates the ECU detects an abnormal drop in circuit impedance and identifies a short circuit fault.

The system enters a safety protection state and disables the airbag to prevent accidental deployment, rendering the airbag inoperative during a collision.

This fault directly affects collision safety protection and requires immediate repair.

The B178B-00 fault code on the Seal U indicates low resistance in the front passenger airbag second-stage inflator circuit, suggesting a possible short circuit. Severity is medium because the SRS may disable the airbag, reducing crash protection. Common causes include a loose, corroded, or moisture-affected airbag connector, or an internal short in the airbag module. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Poor contact or terminal oxidation at the airbag wiring harness connector: The front passenger airbag is inside the instrument panel. Moisture, oxidation, or loosening of the yellow wiring harness connector (usually marked F201 or a similar code) abnormally lowers contact resistance or creates an intermittent short circuit.
  • 2Airbag module internal short circuit: Damaged gas generator squib insulation or an internal circuit short to the housing drops the measured resistance below the standard value (normally 2.0-3.0Ω). This commonly occurs when failing to replace an internally damaged airbag after a collision.
  • 3Wiring harness damaged and shorted to ground: Dashboard removal/installation, aftermarket equipment installation or modification (such as dash cam wiring), or animal chewing damages the harness insulation, creating a short circuit to the vehicle body metal.
  • 4SRS ECU internal detection circuit fault: A damaged ECU internal sampling resistor or A/D conversion circuit causes the resistance sampling value of the second-stage deployment circuit to drift, generating a false fault.
  • 5Non-standard repair operations: Using a standard multimeter to directly measure the airbag connector (use a dedicated airbag resistance tool or ensure test current is <1mA), or installing a non-OEM/refurbished airbag assembly causing a resistance mismatch.
What To Do
  • 1
    Safe power-off and discharge: Turn off the ignition switch, disconnect the battery negative terminal and wait at least 90 seconds (to fully discharge the SRS backup power capacitor). Never connect or disconnect any airbag-related connectors with the power on.
  • 2
    Visual inspection and cleaning: Remove the passenger-side lower dashboard trim panel and glove box. Locate the yellow airbag wiring harness connector (F201). Check for looseness, water ingress, terminal oxidation, or trapped foreign objects. Clean the connector with electrical contact cleaner. Disconnect and reconnect it, verifying the locking tab engages fully.
  • 3
    Airbag assembly resistance measurement: Disconnect the airbag module connector. Measure the resistance between the airbag terminals using a dedicated airbag resistance tester or a digital multimeter (confirm current <1mA). The standard value is 2.0-3.0 Ω. If the resistance is <1.5 Ω or close to 0 Ω, the airbag module has an internal short circuit. Replace the airbag module.
  • 4
    Harness continuity check: Disconnect the SRS ECU connector (located under the center console or at the bulkhead). Measure the resistance to ground of the front passenger airbag stage 2 circuit (usually F201 to the corresponding ECU pin). Resistance must be >1 MΩ. Measure the harness continuity resistance. Resistance must be <1 Ω. If not, repair the damaged harness section.
  • 5
    Parts replacement: If diagnostics confirm an airbag module fault, replace the OEM front passenger frontal airbag assembly (Note: Airbag part numbers for Qin 100/Qin 80 and Qin EV300/450 may differ; verify using the VIN). If the ECU is faulty, replace the SRS control unit.
  • 6
    System configuration and calibration: Connect the VDS2000/VDS3000 diagnostic tool and perform 'Clear DTCs' → 'SRS system self-check' → 'Write configuration parameters' (some models require entering airbag barcode information). Confirm B178B-00 does not reappear and the instrument cluster airbag warning light turns off normally.
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV300: Connector came loose after dashboard removal and refit, causing B178B-00

✓
A 2017 Qin EV300 showed a constant airbag warning light after the dealership replaced the air conditioning evaporator. VDS retrieved DTC B178B-00. The technician found the passenger airbag harness connector (yellow plug) had not fully seated during dashboard removal and installation, and terminal oxidation caused reduced contact resistance. The technician reseated the connector, treated the terminals with electronic cleaner, and cleared the DTC. The system self-test passed and the fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Qin 100 airbag module internal short after collision repair

✓
A 2017 Qin 100 sustained a minor frontal impact. External damage was slight and the airbags did not deploy, but the instrument cluster displayed an airbag fault after repairs. Diagnosis revealed DTC B178B-00; measured resistance on the passenger airbag second stage circuit was only 0.6Ω (normal 2.0–3.0Ω). Disassembly revealed the airbag inflator internal bridge wire had deformed from the impact, causing a short to the housing. Replaced the OEM passenger front airbag assembly (part number: ...verify against vehicle specification) and completed system calibration. Fault resolved.
BYD DTC AI Analysis

Qin EV450 intermittent fault from connector oxidation after water wading

✓
A 2018 Qin EV450 developed an intermittent airbag warning light after driving through deep water in heavy rain; the light sometimes cleared after a restart. A VDS scan retrieved historic fault code B178B-00. Inspection revealed damage to the passenger-side floor harness grommet, letting moisture into the airbag harness connector. This reduced insulation resistance between terminals and created a micro-short. Fix: thoroughly cleaned the connector terminals, dried the harness interior with a heat gun, applied waterproof insulating silicone grease to the connector, and installed a waterproof protective sleeve. The fault has not returned.
BYD DTC AI Analysis

Aftermarket modification pinches wiring harness, causing short on Qin 80

✓
A 2017 Qin 80 owner self-installed a Head-Up Display (HUD), routing the wiring through a dashboard gap and accidentally pinching the passenger airbag harness. After some time, the airbag warning light illuminated with DTC B178B-00. Inspection found the harness insulation chafed against the dashboard metal bracket, causing the second-stage trigger wire to short to ground. The repair involved fixing the damaged harness (replacing the entire section or wrapping locally), repositioning the HUD wiring to avoid the airbag harness, and clearing the fault codes. The system then returned to normal.
Other Seal U Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U overview →
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.