AU
B172211

DTC B172211 indicates the Supplemental Restraint System (SRS) detects an abnormally low-resistance path (short circuit) to body ground in the right knee airbag (passenger-side knee airbag) ignition circuit — Qin Plus

Safety System

DTC B172211 indicates the Supplemental Restraint System (SRS) detects an abnormally low-resistance path (short circuit) to body ground in the right knee airbag (passenger-side knee airbag) ignition circuit.

Under normal conditions, the airbag igniter circuit resistance should be between 2.0 and 3.0 Ω.

The ECU determines a short to ground when it detects circuit resistance below 1.0 Ω or voltage to ground near 0 V.

This fault prevents proper airbag deployment during a collision (as the ECU disables the circuit to protect the power supply) or, in extreme cases, causes unintended deployment.

Simultaneously, the SRS enters fail-safe mode, illuminates the airbag warning light, and may lock the entire airbag system, severely compromising passive safety performance.

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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1Prolonged chafing of the right knee airbag wiring harness against the instrument panel crossmember or glove box bracket damages the insulation, causing the wire to short directly to the metal body.
  • 2Water ingress and oxidation at the airbag connector (yellow double-lock plug) below the right side of the center console forms a short-to-ground path between the pins, particularly after vehicle wading or an air conditioning drain blockage.
  • 3Bridge wire insulation breakdown in the knee airbag module internal squib causes an internal short to ground, usually resulting from airbag module aging or previous external impact.
  • 4During front-end collision repair, the instrument panel frame pinched the wiring harness, or missing retaining clips allowed the harness to shift and contact a ground point.
  • 5SRS ECU internal driver circuit fault (MOSFET breakdown) incorrectly identifying normal low resistance as a short circuit (low probability).
What To Do
  • 1
    Safe power-down: Turn off the ignition, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS backup capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use the BYD VDS diagnostic tool to read fault codes. Confirm B172211 is a current fault (Active), not a history fault, and record the freeze frame data.
  • 3
    Visual inspection: Remove the lower right dashboard trim panel (this usually requires removing the glove box or lower right cover). Inspect the yellow airbag connector for looseness, water ingress, corrosion, or bent pins. Inspect the wiring harness for signs of chafing.
  • 4
    Resistance measurement: Disconnect the knee airbag connector. Use a digital multimeter to measure the resistance to ground on the wiring harness side (ECU side). Normal resistance is greater than 1 MΩ. A resistance close to 0 Ω indicates a short to ground in the wiring harness.
  • 5
    Airbag assembly inspection: Measure the resistance between the two pins on the airbag module side. Standard resistance is 2.0-3.0 Ω. If the resistance is close to 0 Ω, the airbag module has an internal short circuit. Replace the module.
  • 6
    Harness continuity check: If the harness shorts to ground, use the wiring diagram to test the wire from the airbag connector to the SRS ECU (usually located under the center console or center tunnel) for insulation damage causing a ground fault. Carefully inspect the sections where the harness passes through the firewall and the instrument panel frame.
  • 7
    Fault repair: If the wiring harness is damaged, wrap it with high-temperature tape (Kapton) or replace the harness; if the airbag module is faulty, replace the module (Note: Handle the old module as hazardous material).
  • 8
    System reset and verification: Reconnect all connectors (fully engage the yellow plug double lock), connect the battery, and clear the fault codes. Use the diagnostic tool to perform 'SRS system self-diagnosis' or 'igniter circuit check'. Confirm the resistance value returns to normal and no fault codes exist.
Real-World Cases
BYD DTC AI Analysis

Knee airbag connector oxidised and shorted on water-wading vehicle

✓
A 2019 Qin PRO DM developed an airbag warning light that stayed on after wading through water during heavy rain. DTC B172211 was stored. Inspection found obvious water residue inside the right knee airbag connector (located behind the glovebox), with the copper pins oxidized and blackened. The air conditioning drain hose had detached, allowing condensation to accumulate over time. Cleaned the connector pins, dried the wiring harness with compressed air, replaced the AC drain hose, applied conductive grease and reset the system. Fault resolved.
BYD DTC AI Analysis

After accident repairs, the dashboard frame pinched the wiring harness.

✓
A 2018 Qin Pro petrol model developed DTC B172211 one month after front-end collision repairs. Inspection found the right knee airbag harness pinched where it passes through the instrument panel crossbeam mounting bracket. The insulation had chafed through, exposing copper wire, which made direct contact with the vehicle body and created a short to ground. The cause was improper assembly during the accident repair – the harness was not routed to specification. Repaired the wiring by cutting out the damaged section, soldering the joint and sealing with heat-shrink tubing. Rerouted the harness correctly and fitted protective corrugated tubing. Fault cleared.
BYD DTC AI Analysis

Internal short circuit in knee airbag module igniter

✓
A 2020 E3 airbag warning light suddenly illuminated during normal use, logging DTC B172211. Disconnected the airbag connector—harness-side resistance to ground measured normal (infinite). Measured 0.3Ω between pins of the airbag module (lower right of dashboard), indicating an internal short. Replaced the right knee airbag module and used a scan tool to perform coding and configuration (some models require a new airbag ID). Fault resolved. The old module was returned to the manufacturer for analysis, who confirmed an insulation defect in the internal bridge wire.
BYD DTC AI Analysis

Intermittent fault caused by SRS ECU misjudgement

✓
A 2019 E2 intermittently logged DTC B172211, appearing when cold and disappearing when warm. We measured the wiring harness and airbag resistance; both were within normal range. We suspected a fault in the ECU's internal sampling circuit. After replacing the SRS ECU with an identical unit (which required immobiliser and VIN recoding), the fault vanished. Analysis pointed to a drift in the ground-sampling resistor of the ECU's monitoring chip, causing a false short-circuit alert. In similar cases, distinguish between actual wiring shorts and ECU hardware faults.
Other Qin Plus 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.