AU
B1721-00

B1721-00 is a BYD SRS (Supplemental Restraint System) fault code indicating a communication interruption or open circuit between the front passenger (right) knee airbag and the Airbag Control Unit (ACU) — Atto 3

Safety System

B1721-00 is a BYD SRS (Supplemental Restraint System) fault code indicating a communication interruption or open circuit between the front passenger (right) knee airbag and the Airbag Control Unit (ACU).

The fault triggers when the ACU detects the right knee airbag deployment circuit resistance falls outside the normal range (typically 2.0Ω-3.0Ω), causing the system to register a 'not connected' state.

During a collision, the right knee airbag may fail to deploy, leaving the front passenger's knees and lower limbs unprotected.

Additionally, the SRS fail-safe mechanism may force the entire airbag system into a degraded mode, affecting the normal operating logic of other airbags.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Right knee airbag connector loose or disconnected: Previous repairs (such as replacing the cabin air filter or removing and installing the dashboard) left the dedicated yellow connector under the passenger side dashboard incompletely locked, or prolonged driving on rough roads loosened the connector.
  • 2Wiring harness wear or breakage: The knee airbag wiring harness routes behind the front passenger glove box and through the floor harness channel. Excessive compression from the glove box, or detached retaining clips causing long-term friction against metal edges, can cause a short or open circuit.
  • 3Knee airbag module internal fault: Airbag squib open circuit or abnormal resistance. Typical causes include airbag aging, poor internal solder joints, or moisture ingress.
  • 4Airbag Control Unit (ACU) internal fault: Damage to the corresponding knee airbag drive circuit inside the ACU prevents correct circuit resistance detection, or a software logic error causes a false code.
  • 5Poor contact at the body wiring harness to instrument panel wiring harness connector: Oxidised terminals, backed-out pins, or water ingress corrosion at the main and sub harness connection behind the centre console (usually below the front passenger A-pillar or at the centre console frame) interrupt the signal.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment; wear an anti-static wrist strap.
  • 2
    Fault code confirmation: Use the VDS2000 or VDS3100 diagnostic tool to access the SRS system. Read and record the freeze frame data (Freeze Data) for B1721-00. Check if it is a current fault (Present) or a history fault (History). Check for accompanying fault codes (such as B1720-00 left knee airbag fault).
  • 3
    Visual inspection: Remove the passenger-side lower dashboard trim panel (usually requires removing 3-4 T20 screws). Verify the yellow 2-pin connector for the right knee airbag is fully inserted and locked (listen for a 'click'). Inspect the connector for water ingress or burn marks.
  • 4
    Resistance measurement: Disconnect the airbag connector and use a multimeter to measure the resistance across the airbag-side terminals. The normal value is 2.0-3.0Ω. If the multimeter displays OL (open circuit) or the resistance is greater than 10Ω, replace the knee airbag module. If the resistance is normal, measure continuity on the harness side (from the ACU to the connector) and check for an open circuit or a short to ground/power.
  • 5
    Wiring harness repair: If the wiring harness is damaged, repair it using high-temperature tape (Kapton Tape) and install corrugated conduit for protection. If a terminal pin backs out, replace the terminal and verify the contact pressure meets the standard (usually greater than 0.5N).
  • 6
    Component replacement: If testing confirms a faulty knee airbag module, replace it with a genuine part (part numbers vary by model, e.g., 64E-62800-XX for Qin EV). Check and record the new part resistance before installation.
  • 7
    System reset and verification: Reconnect all connectors, reconnect the battery, and use the diagnostic tool to clear the fault code. Perform an ignition cycle self-check (turn IG ON; observe the SRS warning lamp illuminate for 6 seconds and then turn off). Execute the SRS system 'Collision Output Check' and 'Current Fault Check', and confirm B1721-00 no longer appears. Perform a road test, simulating bumpy road conditions to verify no intermittent faults occur.
Real-World Cases
BYD DTC AI Analysis

Qin EV450 SRS light on after accident repair

✓
After a dealership replaced the front bumper and right headlight on a 2018 Qin EV450, the SRS warning light stayed on. VDS showed DTC B1721-00. Inspection found that while removing and refitting the right front fender liner, a technician accidentally pulled the knee airbag wiring harness, dislodging the yellow connector under the passenger-side A-pillar. The connector was plugged back in firmly, its clip locked, and the fault code cleared, fixing the issue. Recommendation: After collision repairs, check all airbag connector conditions strictly per the BYD SRS System Repair Manual.
BYD DTC AI Analysis

Qin 100: Internal open circuit in knee airbag causing intermittent warning

✓
2017 BYD Qin 100. The owner reported the SRS warning light comes on intermittently, particularly during cold starts. The diagnostic tool displayed B1721-00 as an intermittent fault. We measured the right knee airbag resistance at 2.3Ω (normal), but the reading jumped to infinity when lightly tapping the module. Disassembly revealed a cold solder joint on the airbag igniter; thermal expansion and contraction from temperature changes caused poor contact. Replaced the right knee airbag assembly (Part No.: 64E-62800-00), which completely resolved the fault. Recommendation: For intermittent faults, perform a 'Heat & Vibration Test' to simulate operating conditions.
BYD DTC AI Analysis

Knee airbag fault after wading through water — BYD Qin EV300

✓
A 2017 Qin EV300 displayed multiple SRS fault codes on the dashboard after driving through flood water, including B1721-00 and B1720-00. Inspection found water had submerged the passenger-side floor. The knee airbag connector, located at the lowest point of the floor wiring harness, had water ingress that oxidized the pins. After cleaning the connector and blowing the harness dry, B1721-00 remained. Measurement revealed high resistance in the harness due to electrolytic corrosion. Replaced the right front floor wiring harness (including the knee airbag branch) and dried the ACU (to prevent internal short circuits). The fault cleared. Also check the body seals.
BYD DTC AI Analysis

Airbag control unit false positive

✓
2017 BYD Qin 80, no accident history, suddenly set DTC B1721-00. Inspected knee airbag connector and wiring harness – both normal, resistance measured 2.1Ω. Suspected ACU fault, but to rule out interference, first checked for non-OEM accessories (such as dash cam, ambient lighting). Found the owner had installed a 4G router in the passenger glovebox; its power cable was wrapped around the knee airbag harness, causing electromagnetic interference. Removed the interference source and rerouted the wiring (maintaining at least 10cm clearance), fault code did not return. Recommendation: Do not route high-power wiring within 10cm of SRS harnesses.
BYD DTC AI Analysis

Knee airbag connector pin backed out

✓
2018 BYD Qin EV450. DTC B1721-00 appeared at 50,000 km. Found the right knee airbag connector fully seated, but one pin (normally pin 2, ground) had backed out 2 mm from the plastic housing, causing poor contact. Prolonged driving on rough roads with the connector not fully locked likely allowed the pin to work loose. Reinstalled the terminal using a dedicated pin removal tool, ensured the TPA (Terminal Position Assurance) lock fully engaged, and applied conductive paste (not insulating silicone grease) to prevent oxidation. Performed a 100 km road test after repair; fault did not recur.
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.