AU
B171912

DTC B171912 indicates a short to the 12V power supply in the driver-side knee airbag igniter circuit — Seal U

Safety System

DTC B171912 indicates a short to the 12V power supply in the driver-side knee airbag igniter circuit.

In the BYD SRS system, the normal resistance of the airbag igniter is 2.0 ± 0.3 Ω.

The Airbag Control Unit (ACU) continuously monitors this resistance through a low-current diagnostic circuit.

A "short to power" means the signal wire or return wire in the igniter circuit shorts to the vehicle constant power (B+), causing circuit resistance to rise abnormally (approaching infinity or exhibiting short-circuit characteristics).

This fault is extremely dangerous.

It can cause the airbag to deploy unexpectedly while driving, injuring occupants.

Alternatively, during a collision, the ACU may detect the circuit fault and fail to trigger the airbag, disabling the protection function.

Upon detecting this fault, the ACU immediately illuminates the instrument cluster SRS warning light and locks the airbag system into fail-safe mode.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1The knee airbag wiring harness chafes under the steering column or at the instrument panel frame, damaging the insulation and shorting to the body power wiring harness.
  • 2Knee airbag connector (usually located under the dashboard near the centre console): bent pins, backed-out pins, or water ingress causing a short circuit between the power and signal pins.
  • 3Fixing screws or clips pierced the wiring harness during non-standard modifications (such as installing a dash cam, full-coverage floor mats, or scuff plates), causing a short circuit between the cores of a multi-core wire.
  • 4An ignition driver MOSFET breakdown inside the Airbag Control Unit (ACU) shorts the output terminal to the internal power supply.
  • 5Internal squib seal failure in the knee airbag assembly allows moisture ingress, causing the bridge wire to short to the housing.
What To Do
  • 1
    Safe power-down: Switch off the ignition, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the ACU energy storage capacitor and prevent accidental airbag deployment during repair.
  • 2
    Visual inspection: Inspect the wiring harness below the driver's knee and under the instrument panel for wear, scorching, or pinch marks. Focus on areas where the harness contacts the steering column and metal brackets.
  • 3
    Connector check: Disconnect the knee airbag connector (usually under the dashboard, marked yellow). Check pins for bending, corrosion, metal debris, or water stains.
  • 4
    Circuit measurement: Use a multimeter to measure the voltage to ground of the airbag circuit on the wiring harness side. Normal voltage is 0V. A 12V reading confirms a short to power.
  • 5
    Harness continuity test: Measure airbag harness continuity from the connector to the ACU. Check for abnormal continuity with the vehicle power supply lines (constant power or ignition power).
  • 6
    Component isolation diagnosis: Replace the knee airbag with a dedicated airbag simulator (2.7Ω resistor). Power on again and read the fault code. If the fault code changes to "open circuit" or disappears, the airbag assembly is faulty. If the fault code remains a short circuit, the wiring harness or ACU is faulty.
  • 7
    Wiring harness repair: If the wiring harness is damaged, cut out the damaged section, solder a replacement wire of the same color and specification, insulate with double-layer heat-shrink tubing, and adjust the harness routing to prevent further chafing.
  • 8
    Component replacement: If the wiring harness is normal, replace the knee airbag assembly. If the short circuit persists, replace the airbag control unit (reconfigure the vehicle VIN and coding).
  • 9
    System verification: Restore all connections, power on for self-check, clear the fault code using the diagnostic tool, perform the 'SRS System Cycle Test' to confirm the current status is 'Normal', and verify the instrument panel warning light is off.
Real-World Cases
BYD DTC AI Analysis

Qin PRO DM: Knee airbag wiring harness chafed at steering column, causing short circuit

✓
After wading through water, the airbag warning light illuminated on the dashboard. Retrieved DTC B171912. Inspection found damage to the wiring harness protective sleeve under the driver's knee. Internal copper wires were contacting both the steering column mounting bracket (earth) and nearby power wiring simultaneously, creating a short circuit. Cause: Following water ingress, the wiring harness absorbed water and expanded. Long-term friction against the steering column mounting bracket damaged its insulation, and the moisture also corroded the insulation of the adjacent power wire. Repair: Cut out the damaged section of harness (about 15cm), rerouted the harness away from the steering column movement area, and protected it with corrugated tubing. Fault resolved.
BYD DTC AI Analysis

BYD E2 airbag connector water ingress causing pin short circuit

✓
After driving during the wet season, the dashboard displayed an airbag warning. The scan tool showed DTC B171912. Removed and inspected the knee airbag connector located beneath the dashboard near the centre console. Found the sealing ring had deteriorated and detached, allowing rainwater to seep in from the floor and pool inside the connector. This created an electrolytic short between the power and signal pins. Solution: Thoroughly cleaned the connector with electronic contact cleaner, blew it dry, replaced the waterproof sealing ring, and applied conductive anti-corrosion grease. The fault has not recurred.
BYD DTC AI Analysis

BYD E3: Full-surround floor mats pinched the wiring harness, causing a short circuit.

✓
Owner reported the airbag warning light illuminated one week after fitting full-coverage floor mats. Retrieved DTC: left knee airbag short to power. Inspection found the plastic retaining clips used during mat installation were excessively long, penetrating the carpet and piercing the knee airbag wiring harness, causing an internal short between the power wire and the airbag signal wire. Repair: Removed non-compliant clips. Replaced damaged harness section (from main harness branch to connector). Re-routed wiring per specification and secured it with clips away from the floor. Cleared DTCs. System normal.
BYD DTC AI Analysis

Qin EV airbag control unit (ACU) internal drive circuit breakdown

✓
No collision or modification history. The airbag warning light on the instrument panel suddenly came on and stayed on, logging DTC B171912. Measured the knee airbag harness side—no short to power, harness continuity normal, pins OK. Connected an external airbag simulator; the fault code remained. Found a failed power MOSFET in the left knee airbag driver circuit inside the ACU, causing a short between the output and the internal 12V supply. Replaced the airbag control unit assembly, then performed coding, VIN writing and system calibration using a dedicated diagnostic tool. 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.