AU
B178E12

DTC B178E12 indicates a short to power (B+) in the front passenger-side second-stage frontal airbag (PAB Stage 2) ignition circuit — Atto 3

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 — Atto 3
  • 1Front passenger airbag wiring harness insulation wear: Inside the dashboard, A-pillar trim, or floor wiring channel, vibration, chafing, or improper assembly 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 at the front passenger airbag connector (usually located behind the dashboard or glovebox) allows electrolytic corrosion to form a conductive path between terminals after driving through water, high humidity exposure, or an air conditioning condensate leak.
  • 3Clock spring internal short circuit: On vehicles equipped with an integrated clock spring, damaged interlayer insulation on the internal flat cable 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 false detection by the ignition loop monitoring circuit, generating a false short-to-power signal.
  • 5Incorrect wiring after accident repairs: After replacing the dashboard, centre console, or front wiring harness, the technician mistakenly connected the airbag wiring harness in parallel with constant power circuits, such as the instrument illumination power supply or cigarette lighter power supply.
What To Do
  • 1
    Safety preparation and diagnostic confirmation: Use a BYD VDS or Launch X431 diagnostic tool to read all fault codes. Confirm B178E12 is a current fault (not a history fault) and record freeze frame data (including vehicle speed, collision signal status, etc.). Disconnect the battery negative terminal and wait at least 3 minutes for the SRS capacitor to discharge completely.
  • 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 wear, crushing, or burn marks. Focus on the contact points between the wiring harness and the dashboard metal bracket, air conditioning 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 center tunnel or under the center console). Inspect the terminals for signs of water ingress, green copper corrosion, and bent pins. Clean the terminals with compressed air and apply a dedicated electrical contact protectant.
  • 4
    Circuit measurement verification: Set a multimeter to the resistance range. Measure the resistance between the second-stage airbag ignition wires (usually marked PAB Stage 2+ and PAB Stage 2-) at the SRS module connector and vehicle power (B+). The normal reading is infinite (OL). A reading <10 kΩ indicates a short circuit. Test the circuit in sections to locate the short circuit (clock spring, intermediate harness connector, or airbag module itself).
  • 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-test. If the fault disappears, replace the airbag module. If the fault persists, replace the SRS control module or repair the wiring harness.
  • 6
    System verification and calibration: After completing the repair, 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). Confirm 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 Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.