AU
B178E12

DTC B178E12 indicates a short to power (B+) in the front passenger-side second-stage frontal airbag (PAB Stage 2) ignition circuit — Seal U

Safety System

DTC B178E12 indicates a short to power (B+) in the front passenger-side second-stage frontal airbag (PAB Stage 2) ignition circuit.

The BYD SRS system uses a dual-stage airbag ignition design: the first stage provides a low-energy trigger, and the second stage provides a high-energy trigger for varying collision severities.

This DTC indicates an abnormal electrical connection to the vehicle power circuit within the wiring harness between the SRS control module and the front passenger airbag second-stage inflator (resistance below threshold, typically <2Ω).

This short circuit may cause: 1) unintended airbag deployment (extremely dangerous); 2) airbag deployment failure during a collision (due to bypassed current); 3) the SRS system to enter protection mode, disabling all airbag functions.

The BYD diagnostic protocol identifies the fault suffix '12' as 'Short to Battery+'.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Front passenger airbag wiring harness insulation wear: Vibration, chafing, or improper assembly inside the dashboard, A-pillar trim, or floor wiring channel damages the harness outer sheath, causing a short circuit to a power wire (such as constant 12V+).
  • 2Airbag connector water ingress or corrosion: Poor sealing of the front passenger airbag connector (usually located behind the dashboard or glovebox). Vehicle wading, high-humidity environments, or air conditioning condensate leaks cause electrolytic corrosion between the terminals, forming a conductive path.
  • 3Clock spring internal short circuit: On vehicles equipped with an integrated clock spring, damaged insulation between the internal flat cable layers shorts the airbag ignition circuit to the horn/multifunction switch power wire.
  • 4SRS control module internal driver circuit fault: Internal breakdown of the airbag driver chip (such as a dedicated NXP or Infineon IC) or a false detection by the ignition loop monitoring circuit generates a false short-to-power signal.
  • 5Incorrect wiring after accident repairs: After replacing the dashboard, centre console, or front wiring harness, technicians incorrectly connected the airbag wiring harness in parallel with permanent live circuits, such as the instrument panel illumination power supply or cigarette lighter power supply.
What To Do
  • 1
    Safety preparation and diagnostic confirmation: Use the BYD VDS or Launch X431 diagnostic tool to read all fault codes. Confirm B178E12 is a current fault (not a historical fault) and record the freeze frame data (including vehicle speed, collision signal status, etc.). Disconnect the battery negative terminal and wait at least 3 minutes to ensure the SRS capacitor fully discharges.
  • 2
    Wiring harness visual inspection: Remove the passenger-side lower dashboard trim panel, glove box, and lower A-pillar trim panel. Carefully inspect the airbag wiring harness (usually wrapped in yellow corrugated conduit) for signs of abrasion, crushing, or burning. Focus on the contact points between the wiring harness and the dashboard metal bracket, A/C duct, and sharp body edges.
  • 3
    Connector inspection: Disconnect the front passenger airbag module connector (located behind the dashboard; remove the retaining bolts) and the SRS control module connector (usually located in the centre tunnel or under the centre console). Check the terminals for signs of water ingress, green copper corrosion, or bent pins. Clean the terminals with compressed air and apply a dedicated electrical contact protectant.
  • 4
    Circuit measurement verification: Using a multimeter set to resistance, measure the resistance between the second-stage airbag ignition wires at the SRS module connector (usually marked PAB Stage 2+ and PAB Stage 2-) and the vehicle power supply (B+). The normal reading is infinity (OL). A reading <10kΩ indicates a short circuit. Test the circuit in sections to locate the short circuit (clock spring, intermediate harness connector, or airbag module).
  • 5
    Component replacement test: If the wiring harness tests normal, replace the original airbag module with a known-good front passenger airbag simulator (resistance 2 Ω ± 0.5 Ω). Clear the fault code and power on for a self-check. If the fault disappears, replace the airbag module. If the fault remains, replace the SRS control module or repair the wiring harness.
  • 6
    System verification and calibration: After completing repairs, reconnect all connectors and the battery. Use the diagnostic tool to perform 'SRS system self-diagnosis' and 'configuration reset' (write the VIN if replacing the control module). Verify the fault code clears and does not recur, and check that the airbag warning light turns off after 6 seconds.
Real-World Cases
BYD DTC AI Analysis

Qin EV passenger-side airbag harness corroded and shorted after water ingress

✓
2019 Qin EV. Owner reported the airbag warning light stayed on after the vehicle drove through water. DTC B178E12 (current fault) present. Removed the passenger glovebox and found water in the airbag connector; terminals were oxidised green. Further inspection showed the AC drain hose was blocked, causing condensate to drip onto the airbag harness connector below the blower motor. Cleaned the terminals with electronic contact cleaner, replaced the connector seal, and cleared the AC drain hose. Fault resolved.
BYD DTC AI Analysis

E2 aftermarket navigation installation caused airbag harness misconnection to power

✓
A 2020 BYD E2 airbag warning light illuminated the day after an aftermarket head unit installation at a third-party workshop. Diagnostics revealed DTCs B178E12 and B178D12 (passenger airbag first stage short to power). Inspection found the installer had mistaken the passenger airbag wiring harness (yellow connector) for the original radio constant power wire, splicing it into the 12V supply. Technicians corrected the wiring connections, repaired the damaged insulation on the airbag harness, and reset the SRS module configuration.
BYD DTC AI Analysis

E3 clock spring internal inter-layer short circuit

✓
A 2020 BYD E3 had an intermittent airbag warning light when turning the steering wheel to specific angles. Scanned code B178E12 (intermittent). Checked the passenger airbag harness with no issues found. Found the integrated clock spring under the steering wheel had an internal flat cable with an interlayer short; turning the wheel caused the airbag circuit to intermittently short to the horn power supply. Replaced the clock spring assembly. Fault resolved.
BYD DTC AI Analysis

SRS module internal fault after accident repair

✓
A 2019 Qin EV showed an airbag fault on the instrument cluster after front collision repairs (front bumper and condenser replaced; airbags did not deploy). The scan tool read DTC B178E12, which would not clear. The passenger airbag harness measured infinite resistance to power; the harness was normal. The fault persisted with an airbag simulator. The SRS control module had sustained impact damage during the collision, damaging the internal second-stage ignition drive circuit. Replaced the SRS control module and completed online programming and matching; fault resolved.
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.