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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.

5
Cases Logged
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.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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.