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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) — Seal 6 EV

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 Seal 6 EV indicates a communication interruption or open circuit in the front passenger right knee airbag circuit. This is a medium-severity SRS issue because 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 inspection and repair. Typical repair cost data is available.
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Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Right knee airbag connector loose or disconnected: The dedicated yellow connector under the passenger dashboard remains incompletely locked after previous repairs (such as replacing the air conditioning filter or removing and installing the dashboard), or prolonged driving on rough roads caused it to loosen.
  • 2Wiring harness worn or broken: The knee airbag wiring harness routes behind the front passenger glovebox and through the floor harness channel. Excessive compression from the glovebox, detached harness retaining clips, or prolonged friction against metal edges can cause a short or open circuit.
  • 3Knee airbag module internal fault: Internal squib open circuit or abnormal resistance. Typical causes include airbag unit 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 triggers a false code.
  • 5Poor contact at the connector between the body wiring harness and the instrument panel wiring harness: At the main and auxiliary harness joint at the rear of the center console (usually below the passenger A-pillar or at the center console frame), pin oxidation, backed-out pins, or corrosion from water ingress causes signal interruption.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds for the SRS capacitor to fully discharge to prevent accidental airbag deployment; wear an anti-static wrist strap.
  • 2
    Fault code confirmation: Use a 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). Check that the yellow 2-pin right knee airbag connector 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 reading is OL (open circuit) or the resistance exceeds 10Ω, 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 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 pin backs 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., 64E-62800-XX for Qin EV). Check and record the new part's resistance 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-check (turn IG ON and verify the SRS warning light illuminates for 6 seconds and then turns off). Execute the SRS system "Collision Output Check" and "Current Fault Check" to confirm B1721-00 does not return. Perform a road test over bumpy surfaces 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 Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.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 Seal 6 EV →Seal 6 EV 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.