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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) — Qin Plus

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.

The B1721-00 fault code on the Qin Plus indicates a medium-severity SRS fault involving an open circuit or communication interruption between the front passenger right knee airbag and the Airbag Control Unit. The knee airbag may not deploy in a crash. Common causes include a loose yellow connector under the passenger dashboard or damaged wiring near the glovebox. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Right knee airbag connector loose or disconnected: Previous repairs (such as replacing the A/C filter or removing and installing the dashboard) may have left the dedicated yellow connector under the passenger dashboard unlocked, or prolonged driving on rough roads may have loosened the connection.
  • 2Wiring harness wear or breakage: The knee airbag wiring harness runs behind the front passenger glovebox and through the floor harness channel. Excessive pressure from the glovebox or a detached harness retaining clip allowing long-term friction against metal edges may cause a short or open circuit.
  • 3Knee airbag module internal fault: Airbag squib open circuit or abnormal resistance. Common causes include airbag aging, poor internal solder joints, or moisture ingress.
  • 4Airbag Control Unit (ACU) internal fault: Damage to the corresponding knee airbag driver circuit inside the ACU prevents accurate circuit resistance detection, or a software logic error triggers a false DTC.
  • 5Poor contact at the body wiring harness to instrument panel wiring harness connector: Located at the main and secondary harness junction behind the center console (usually below the passenger A-pillar or at the center console frame). Pin oxidation, backed-out terminals, or water corrosion interrupts the signal.
What To Do
  • 1
    Safety preparation: Disconnect the negative battery 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. Read and record the freeze frame data (Freeze Data) for B1721-00. Check if the fault is current (Present) or historical (History), and 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 2.0-3.0 Ω. If the multimeter displays OL (open circuit) or the resistance exceeds 10 Ω, replace the knee airbag module. If the resistance is normal, measure continuity on the harness side (ACU to connector) to check for an open circuit or a short to ground/power.
  • 5
    Wiring harness repair: If the wiring harness is damaged, repair it with high-temperature tape (Kapton Tape) and install corrugated conduit for protection. If a pin has backed out, replace the terminal and verify the contact pressure meets the standard (usually greater than 0.5N).
  • 6
    Component replacement: If diagnostics confirm a faulty knee airbag module, replace it with a genuine part (part numbers vary by model; e.g., Qin EV: 64E-62800-XX). Check and record the resistance of the new part before installation.
  • 7
    System reset and verification: Reconnect all connectors, reconnect the battery, and clear the fault code using the diagnostic tool. Perform an ignition cycle self-test (turn IG ON and observe the SRS warning lamp; it should illuminate for 6 seconds and then turn off). Execute the SRS system "Collision Output Check" and "Current Fault Check" to confirm B1721-00 does not return. Perform a road test, simulating bumpy road conditions to ensure 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 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus overview →
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.