AU
B17211B

DTC B17211B indicates the Supplemental Restraint System (SRS) detects a communication loss or abnormal resistance between the right knee airbag (typically located under the passenger-side dashboard) and the SRS control module — Atto 8

Safety System

DTC B17211B indicates the Supplemental Restraint System (SRS) detects a communication loss or abnormal resistance between the right knee airbag (typically located under the passenger-side dashboard) and the SRS control module.

Specifically, this fault indicates an open airbag igniter circuit (resistance too high, typically >10Ω), which the system registers as a "not connected" state.

This condition prevents the right knee airbag from deploying during a collision and continuously illuminates the airbag fault warning lamp.

This constitutes a hard fault or intermittent open circuit requiring immediate repair to ensure passive safety system integrity.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Knee airbag wiring harness connector loose or disconnected: Following vehicle repair, modification, or collision repair, the yellow SRS connector under the instrument panel does not fully lock, or a damaged locking mechanism causes poor contact.
  • 2Airbag igniter internal open circuit: Broken internal heating wire in the right knee airbag inflator (due to aging, moisture, or minor impact damage), resulting in infinite circuit resistance.
  • 3Harness wear or breakage: Vibration and friction during long-term use damage the insulation and break the copper core of the wiring harness under the dashboard, especially where it passes over metal bracket edges.
  • 4SRS control module internal fault: Damage to the control module internal detection circuit triggers a false airbag open-circuit fault, usually accompanied by other related fault codes.
  • 5Water ingress or corrosion: After the vehicle drives through water, water enters and oxidizes the SRS wiring harness connector near the floor. This causes terminal corrosion and poor contact, resulting in resistance exceeding the normal range (2.0-3.0Ω).
What To Do
  • 1
    Safety preparation: Park the vehicle on a level surface, turn the ignition switch to the OFF position, disconnect the negative battery cable, and wait at least 90 seconds to completely discharge the SRS system backup power supply and prevent accidental airbag deployment.
  • 2
    Fault Confirmation: Use the BYD dedicated diagnostic tool (VDS or ED400) to read the fault code. Confirm B17211B is a current fault (Current), not a history fault (History). Read the data stream to check the airbag resistance value.
  • 3
    Visual inspection: Remove the front passenger lower dashboard trim panel and locate the right knee airbag assembly (yellow connector). Verify the connector is fully inserted and locked. Confirm no visible damage, water ingress, or looseness.
  • 4
    Resistance measurement: Disconnect the airbag connector and use a dedicated SRS resistance measuring tool (or a digital multimeter, low-resistance range only) to measure the resistance between the airbag-side terminals. The standard value is 2.0-3.0Ω. If the tool displays OL or ∞, replace the airbag assembly.
  • 5
    Harness continuity test: Disconnect the SRS control module connector (located under the centre console or inside the centre armrest). Measure harness continuity between the control module end and the airbag connector end. Verify no open circuit or short to ground.
  • 6
    Substitution check: If the wiring harness is normal, connect a known-good knee airbag or a dedicated simulation resistor (2.7Ω) to the wiring harness. Clear and re-read the fault code to confirm if the fault code changes, determining whether the fault is in the airbag or the circuit.
  • 7
    Repair and verification: Replace the faulty component (airbag assembly) or repair the wiring harness. Apply conductive grease to all connectors and lock them fully. Reconnect the battery. Turn the ignition switch to the ON position to perform the SRS system self-check. Verify the fault light turns off and the diagnostic tool shows no fault codes.
Real-World Cases
BYD DTC AI Analysis

Unlocked connector after accident repair caused airbag warning light to illuminate.

✓
After front collision repairs, the airbag warning light on a 2019 BYD Qin EV stayed on. The scan tool retrieved DTC B17211B. Inspection found that after replacing the right knee airbag, the technician had not fully seated the yellow 2-pin connector. Only one click was heard, and the secondary lock did not engage. After reseating the connector until a double click was heard and clearing the fault code, the system returned to normal. This case shows the importance of double-locking SRS connectors.
BYD DTC AI Analysis

Knee airbag internal open circuit replacement case

✓
2020 BYD E2, 30,000 km, no accident history. Airbag warning light came on suddenly. Read DTC B17211B; live data showed infinite resistance on the right knee airbag. Removed and inspected the connector—found it normal. Measured the airbag body resistance: OL (open circuit). Replaced the right knee airbag assembly with an OEM unit (P/N: C#E2-5820000). New airbag measured 2.4 Ω. After installation, the fault cleared. Suspect the heating wire inside the inflator fractured due to premature aging.
BYD DTC AI Analysis

Dashboard wiring harness wear caused an intermittent open circuit

✓
A 2019 BYD E3 exhibited an intermittent airbag warning light that became more frequent on rough roads. A scan tool read DTC B17211B as an intermittent fault. Removing the lower dash panels revealed the right knee airbag wiring harness chafing where it passed through a metal bracket. The retaining clip had detached, and long-term friction against the bracket edge fractured three copper cores, leaving only a partial connection. Repaired the harness by soldering and applying heat-shrink tubing, then rerouted and secured it to avoid contact with sharp edges. This completely resolved the fault.
BYD DTC AI Analysis

Repaired corroded SRS connector on water-exposed vehicle

✓
BYD Qin EV presented DTC B17211B and multiple airbag fault codes after water damage repairs. Inspection found severe water stains and green copper corrosion on the right knee airbag connector (located at the floor wiring harness branch). We cleaned the terminals with electronic contact cleaner, sanded the oxidation layer with fine sandpaper, applied special conductive grease, and replaced the waterproof sealing ring. The airbag itself had no water ingress and its resistance was normal. After repairs, we cleared the fault codes and the system passed self-diagnosis. Always check the sealing integrity of all SRS connectors on any water-damaged vehicle.
Other Atto 8 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.