AU
B17871B

This DTC indicates an open circuit or high resistance fault in the second-stage firing circuit of the driver-side dual-stage airbag — Seal U

Safety System

This DTC indicates an open circuit or high resistance fault in the second-stage firing circuit of the driver-side dual-stage airbag.

In modern passive safety systems, a dual-stage airbag inflates in stages according to collision severity: the first stage provides basic cushioning, and the second stage adds inflation during a severe collision to optimize protection.

The "1B" sub-code typically indicates the SRS control module detects second-stage trigger circuit resistance outside the calibrated range (normal: 1.5-3.0 Ω).

Possible causes include a disconnected connector, a wiring harness open circuit, an internal open circuit in the clock spring (spiral cable), or a faulty airbag module.

During a high-speed frontal collision, this fault causes the airbag to deploy in single-stage mode only.

This reduces head and chest protection but typically does not affect normal first-stage airbag operation.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Driver airbag module connector loose or making poor contact (common after removing the steering wheel or replacing the airbag without fully locking the yellow plug)
  • 2Open circuit in the clock spring (spiral cable) internal second-stage circuit, especially after frequent steering wheel rotation breaks the internal flat cable.
  • 3The airbag wiring harness insulation near the steering column wears due to long-term steering movement, causing wire breakage or intermittent poor contact.
  • 4Corrosion, looseness, or terminal back-out in the ground circuit between the SRS control module and the airbag, causing abnormally high circuit resistance.
  • 5Open circuit in the second-stage igniter (squib) inside the airbag module (usually occurs with an internal diagnostic resistor fault)
What To Do
  • 1
    Use a genuine BYD diagnostic tool (VDS or ED400) to read all SRS fault codes. Check for accompanying B1787-00 (first-level fault) or B1700 series fault codes, and record the resistance value in the freeze frame data.
  • 2
    Perform the safe power-down procedure: disconnect the 12V battery negative terminal, wait at least 90 seconds to ensure the SRS backup capacitor discharges fully, then remove the driver-side airbag module (Caution: do not measure the airbag pins directly with a multimeter).
  • 3
    Verify the yellow airbag module connector (usually marked DAB-Stage2) is fully inserted and clicks into place. Measure the resistance between the Stage 2 deployment pins (standard value 1.6-2.1 Ω). Check the pins for oxidation, push-out, or deformation.
  • 4
    Remove the steering wheel and inspect the clock spring (spiral cable) body and the 6-pin or 8-pin connector beneath it. Use a multimeter to measure continuity of the clock spring's corresponding second-stage circuit. Turn the steering wheel while monitoring the resistance for an intermittent open circuit (>10Ω).
  • 5
    Inspect the wiring harness along the steering column shroud, focusing on wear, crushing, or breakage at the wiring harness retaining bracket near the universal joint. If necessary, open the corrugated conduit to inspect the internal wires.
  • 6
    If the wiring harness and connectors are normal, replace the clock spring assembly (Note: Some models require steering angle sensor calibration). If the fault persists, measure the wiring harness between the SRS ECU and the clock spring for continuity and insulation to ground.
  • 7
    Reassemble all components and ensure the airbag connector is fully locked. Connect the battery and clear the fault code. Perform the SRS system self-check cycle (cycle the ignition switch ON-OFF three times). Confirm B17871B does not return and the airbag warning light turns off normally.
Real-World Cases
BYD DTC AI Analysis

Airbag connector not reseated after recall repair

✓
The vehicle arrived with the airbag light on. Found DTC B17871B. The owner reported an airbag recall was performed at another shop one week earlier. Removed the steering wheel and found the new driver airbag second-stage connector (yellow) was not fully seated and the locking tab had not engaged. Reseated the connector and confirmed the lock engaged. Cleared the fault code; it did not return. Suspected cause: previous technician did not verify connector lock status per standard procedure.
BYD DTC AI Analysis

Clock spring internal fracture causing intermittent fault

✓
The BYD Qin EV owner reported the airbag warning light illuminated intermittently, particularly going out after turning the steering wheel. The scan tool stored history fault code B17871B. Removing the combination switch assembly revealed the clock spring (spiral cable) internal second-stage circuit opened after approximately two turns. Replacing the clock spring assembly and calibrating the steering wheel angle sensor cleared the fault. Prolonged stationary steering caused fatigue fracture of the internal flat cable.
BYD DTC AI Analysis

Steering column wiring harness protective sleeve chafed through, short circuit

✓
E2 showed intermittent airbag warning light flashing on rough roads; static tests normal. Removed and inspected steering column lower harness. Found airbag harness insulation worn through at universal joint mounting bracket from rubbing, second-stage circuit wire nearly severed with unstable contact resistance. Repaired harness (soldered connections with heat-shrink insulation), rerouted and secured harness, added anti-chafe sleeve. Fault resolved.
BYD DTC AI Analysis

Water ingress corroded the airbag connector.

✓
Vehicle drove through water exceeding the door sill height. Two weeks later the airbag warning light illuminated with DTC B17871B. The connector below the driver's airbag (inside the lower steering wheel trim panel) showed water staining and green corrosion. Oxidised pins caused contact resistance >5Ω. Cleaned the pins, sprayed electronic contact cleaner, applied conductive grease, and reconnected the connector. Fault cleared. Check the door seals and drain holes.
BYD DTC AI Analysis

Airbag module coding error after accident repair

✓
Fitting a used airbag module to a repaired accident vehicle set DTC B17871B. The replacement module did not match the vehicle configuration—a single-stage unit was installed in a dual-stage system. The E2 base model uses single-stage airbags, while the high-spec variant uses dual-stage units; the incorrect module meant the SRS ECU could not recognise the second-stage circuit. Replacing the driver airbag assembly with the correct part number cleared the fault and the system passed its self-check.
Other Seal U 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: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.