AU
B171E1B

This fault code indicates the airbag control unit (SRS ECU) detects the left knee airbag (Driver Knee Airbag) igniter circuit resistance exceeds the calibrated upper limit (typically >3 — Seal U

Safety System

This fault code indicates the airbag control unit (SRS ECU) detects the left knee airbag (Driver Knee Airbag) igniter circuit resistance exceeds the calibrated upper limit (typically >3.5Ω; normal range is 2.0-3.0Ω).

Excessive resistance indicates a high-resistance point in the igniter circuit.

Potential causes include poor connector contact, a partially broken wiring harness, oxidation, or an aging spiral resistance wire inside the airbag module.

This fault forces the SRS system into a degraded protection mode, and the left knee airbag may fail to deploy during a collision (this airbag protects the driver’s lower limbs from rearward steering column movement or instrument panel intrusion).

The airbag warning light remains illuminated on the instrument panel, and some models trigger a warning buzzer.

Because this fault affects a core passive safety system function, it presents a safety risk and requires immediate inspection.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Knee airbag wiring harness connector (usually a yellow plug behind the driver-side knee trim panel) is loose, has backed-out terminals, or has oxidized pins, causing increased contact resistance.
  • 2Broken igniter spiral resistance wire inside the airbag module, or dry solder joint (component aging or production batch defect)
  • 3Physical damage to the wiring harness: Kicking wears the wiring harness in the driver’s footwell, or the wiring harness below the steering column rubs against a metal bracket, partially breaking the copper strands.
  • 4SRS ECU internal sampling circuit reference drift or A/D converter fault (less common; cross-check to rule out false ECU fault)
  • 5After vehicle wading or washing, water enters the knee airbag connector and corrodes the terminals, forming an oxide film that causes high resistance.
What To Do
  • 1
    Safe power-off: Switch off the ignition, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS backup power supply and prevent accidental airbag deployment during repair.
  • 2
    Visual inspection: Remove the driver-side lower trim panel (knee bolster). Verify the yellow airbag connector is fully inserted and the locking tab is engaged. Inspect for signs of water ingress or green oxidation on the pins.
  • 3
    Resistance measurement: Use a digital multimeter (verify it is set to the resistance range with no current output, or use a dedicated airbag resistance measuring tool) to measure the resistance across the airbag igniter terminals. Normal value: 2.0-3.0Ω; if >3.5Ω, the resistance is too high.
  • 4
    Sectional isolation check: Disconnect the airbag module connector. First, measure the wiring harness side resistance (should be <1Ω, insulation to ground >1MΩ). Next, measure the airbag module resistance to pinpoint whether the fault is in the wiring harness or the module.
  • 5
    Repair: If the connector is oxidized, clean with electrical contact cleaner and apply conductive grease; if the wiring harness has broken strands, solder the repair and install double-wall heat-shrink tubing; if the module is faulty, replace the left knee airbag assembly.
  • 6
    System reset and verification: After restoring the connection, turn on the power. Use the VDS2000 or a dedicated BYD diagnostic tool to clear the fault code. Perform 'SRS system self-learning' or 'configuration coding'. Finally, conduct a road test to confirm the fault code does not return and the airbag indicator turns off normally.
Real-World Cases
BYD DTC AI Analysis

BYD Qin Pro DM airbag fault after wading repairs

✓
A 2019 BYD Qin PRO DM was towed to the workshop after being submerged in floodwater up to seat height during heavy rain. The instrument cluster showed a constant airbag warning light. The technician read DTC B171E1B (current fault). Removing the driver's knee panel revealed water inside the airbag connector with oxidised black pins. The technician cleaned the connector pins with anhydrous alcohol, blew them dry, applied special conductive grease, and refitted the component. Resistance returned to 2.3 Ω. After clearing the fault code, the system returned to normal. Also check the floor drain plugs for blockages and inspect the wiring loom grommets for integrity.
BYD DTC AI Analysis

A loose connector after an accident repair caused a false alarm.

✓
After a front-end collision, I replaced the dashboard assembly. Once reassembled and started, the vehicle immediately threw code B171E1B. I checked and found the knee airbag connector was plugged in, but the locking tab hadn’t fully engaged (I heard the click, but the secondary lock hadn’t closed). The SRS connector has an internal shorting bar. When the connector isn’t fully seated, it doesn’t push the shorting bar open completely, causing abnormal circuit resistance. I reconnected the connector and confirmed the two-stage locking mechanism had fully engaged. The fault code then automatically turned into a history code. I cleared it, and it never returned.
BYD DTC AI Analysis

Wiring harness chafing caused intermittent high resistance.

✓
The owner reported that the airbag warning light occasionally illuminated, especially when braking or moving the left leg. The scanner retrieved historical fault code B171E1B. Inspection found the knee airbag wiring harness under the steering column was interfering with a metal bracket. The worn insulation caused some copper strands to fracture (only a few remained connected), creating intermittent high resistance. Repair: cut the damaged harness section, extended it with same-specification high-temperature wire using soldered joints, re-routed it with corrugated tubing for protection, and secured to the original harness clips to prevent future interference. Fault fully resolved.
BYD DTC AI Analysis

Batch Replacement Case: Internal Defect in Airbag Module

✓
A 2018 Qin Pro petrol variant showed current fault code B171E1B during PDI. The vehicle had no accident history or water ingress. Disconnected the harness and measured the airbag module body resistance directly: 4.8 Ω. This confirmed a manufacturing defect in the module's internal igniter spiral resistor. Replaced the left knee airbag assembly (part number must match ECU software version) and the resistance returned to normal. Later checked the manufacturer's technical bulletin — this batch has process issues. For similar faults, measure the module body resistance first to distinguish between harness and module failures.
BYD DTC AI Analysis

Upgrade resolves false alarm caused by overly strict ECU software threshold.

✓
Multiple 2018 Qin PRO vehicles logged DTC B171E1B, but measured knee airbag resistance on all vehicles was 3.2–3.4 Ω (slightly above standard, but within normal ageing range). Wiring harness and connector inspection found no faults. BYD technical support confirmed the cause as early SRS ECU software with overly tight sampling thresholds (threshold set to 3.0 Ω, should be 3.8 Ω). Flashed the ECU to the latest software using VDS2000 (version must be above V3.2.1). After flashing, the DTC cleared automatically and has not returned. No hardware needed replacement.
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.