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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 U

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 U indicates a medium-severity SRS fault involving an open circuit or communication interruption between the front passenger knee airbag and the Airbag Control Unit. This may stop the knee airbag deploying in a crash. Common causes include a loose yellow connector under the passenger dashboard or damaged wiring near the glove box. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Right knee airbag connector loose or disconnected: The dedicated yellow connector under the passenger side dashboard is not fully locked following previous repairs (such as replacing the A/C filter or removing and installing the dashboard), or prolonged driving on rough roads loosened the connection.
  • 2Worn or broken wiring harness: The knee airbag wiring harness routes behind the passenger glove box and through the floor harness channel. Excessive compression from the glove box, detached harness retaining clips, or long-term friction against metal edges can cause a short or open circuit.
  • 3Knee airbag module internal fault: Airbag 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 internal knee airbag drive circuit prevents the ACU from correctly detecting circuit resistance, or a software logic error triggers a false DTC.
  • 5Poor contact at the body wiring harness to instrument panel wiring harness connector: Terminal oxidation, backed-out pins, or water corrosion at the main-to-sub harness connection behind the centre console (usually below the passenger A-pillar or at the centre console frame) causes signal interruption.
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 enter the SRS system. Read and record the freeze frame data (Freeze Data) for B1721-00. Check if the fault is current (Present) or historical (History). Check for accompanying fault codes (such as B1720-00 left knee airbag fault).
  • 3
    Visual inspection: Remove the front passenger lower dashboard trim panel (usually requires removing 3-4 T20 screws). Verify 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 multimeter reads OL (open circuit) or the resistance exceeds 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 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 tubing 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., 64E-62800-XX for Qin EV). Check and record the resistance of the new part 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 and observe the SRS warning lamp; it must illuminate for 6 seconds and then turn off). Perform 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 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 U Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.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.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.