AU
B1718

B1718 is a BYD-specific SRS (Supplemental Restraint System / airbag system) fault code indicating an unintended low-resistance path (short to ground) between the driver-side knee airbag (Left Knee Airbag) ignition circuit and vehicle ground (GND) — Seal U

Safety System

B1718 is a BYD-specific SRS (Supplemental Restraint System / airbag system) fault code indicating an unintended low-resistance path (short to ground) between the driver-side knee airbag (Left Knee Airbag) ignition circuit and vehicle ground (GND).

The knee airbag contains an igniter (squib).

Under normal conditions, both igniter terminals must show an open circuit (infinite resistance) to ground.

A short to ground indicates current may bypass the igniter and flow directly to ground, causing the airbag control unit (ACU) to register a fault in that airbag circuit.

This fault poses a severe safety risk.

The short circuit can prevent the airbag from deploying during a collision or, in extreme cases, trigger unintended deployment.

The system classifies this fault as severity level 3, illuminates the SRS warning lamp continuously, and may force the entire airbag system into fail-safe mode (disabling all airbags).

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Knee airbag wiring harness insulation damage: Prolonged friction or compression (e.g., driver knee impact or improper aftermarket pedal installation) damages the wiring harness below the steering wheel or inside the dashboard, breaking the outer sheath and causing the copper wire to contact the vehicle body metal.
  • 2Connector water ingress or corrosion: Water enters the knee airbag connector (usually located under the left side of the dashboard) due to vehicle wading, A/C condensate leaks, or car washing, causing a short circuit between terminals or between a terminal and the housing.
  • 3Airbag Control Unit (ACU) internal fault: A damaged ACU internal drive or monitoring circuit triggers a false short-to-ground fault (actual wiring is normal).
  • 4Internal short circuit in knee airbag assembly: Internal short circuit in airbag igniter (very rare; typically causes abnormally low airbag resistance)
  • 5Improper repair work: screws crushed or clips pierced the wiring harness during previous removal of the instrument panel or knee bolster, or improper securing during reassembly caused the harness to contact sharp metal edges.
What To Do
  • 1
    Safety preparation: Turn off the ignition, disconnect the 12V battery negative terminal, and wait at least 90 seconds (to fully discharge the SRS capacitor and prevent accidental airbag deployment).
  • 2
    Visual inspection: Remove the driver-side knee trim panel (Under Cover). Check the knee airbag wiring harness (marked by a yellow sleeve) for signs of crushing, cuts, or abrasion. Focus on contact points with the steering column and metal brackets.
  • 3
    Connector inspection: Disconnect the knee airbag connector (usually protected by a shorting bar/shorting spring). Check the terminals for signs of water ingress, corrosion, burning, or deformation. Measure the resistance between each of the two harness-side terminals and body ground. Normal resistance is infinity (OL). A resistance of less than 1 Ω indicates a short circuit.
  • 4
    Wiring harness insulation test: Use a megohmmeter or a multimeter on the high-resistance range to measure the insulation resistance to ground of the knee airbag circuit (from the airbag connector to the ACU connector). The resistance must be greater than 10MΩ.
  • 5
    Airbag unit test: After confirming normal wiring, measure the resistance between the two terminals of the knee airbag assembly (standard value is typically 2.0-3.0Ω). Separately measure the resistance between each terminal and the airbag metal housing (should be infinite). If continuity exists between a terminal and the housing, replace the airbag assembly.
  • 6
    Fault repair: Replace the damaged wiring harness or repair the insulation (seal tightly with heat-shrink tubing), clean or replace corroded connectors, and secure the wiring harness retaining clips in place to prevent interference with the vehicle body.
  • 7
    System verification: Reconnect all components and the battery. Use the diagnostic tool to clear the fault code. Perform the SRS system self-test (typically involves cycling the ignition switch). Verify DTC B1718 does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Corroded knee airbag connector on water-damaged vehicle caused B1718

✓
After driving through heavy floodwater, a 2020 BYD Qin petrol's SRS warning light stayed on. Scanning revealed DTC B1718 (left knee airbag short to ground). The technician removed the driver's knee panel and found significant water staining and copper oxide corrosion inside the knee airbag connector under the left side of the steering column. The connector seal had deteriorated, allowing water to enter the terminal cavity during the flood and causing the terminals to short against the grounded connector housing. The technician cleaned the connector thoroughly with electronic contact cleaner, dried it, polished the terminals with fine sandpaper, applied conductive grease, and fitted a new waterproof seal. Reinstallation cleared the fault.
BYD DTC AI Analysis

Aftermarket metal pedal interference caused wiring harness wear

✓
After the owner of a 2021 BYD Tang DM installed aftermarket metal pedals, the airbag warning light occasionally illuminated while driving, and the diagnostic tool displayed DTC B1718. Inspection revealed the mounting screws for the modified pedals were too long and had pierced the knee airbag wiring harness (yellow harness) beneath the dashboard, causing the copper wires to intermittently short to chassis ground. Solution: Removed the non-compliant pedals. Repaired the damaged harness—cut out the damaged section, soldered the wires, and insulated with double-layer heat-shrink tubing. Re-routed the wiring along the standard path and secured it with OEM clips, ensuring adequate clearance from the pedal mechanism.
BYD DTC AI Analysis

Wiring harness incorrectly secured after accident repair caused short circuit

✓
A 2019 BYD Yuan EV returned one week after delivery following a front collision repair (dashboard replaced), with the airbag warning light on and DTC B1718 stored. Workshop inspection found that during reassembly, the technician had incorrectly trapped the knee airbag wiring harness under the dashboard metal bracket. The bracket’s sharp edge cut through the harness, exposing the internal wires and causing a short to ground. Replaced the damaged knee airbag harness assembly (too damaged to repair). Re-routed the wiring strictly according to the workshop manual wiring diagram, using all OEM retaining clips to prevent the harness from contacting any metal edges during vehicle vibration.
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.