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 — Atto 8

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.

The B171E1B fault code on the Atto 8 indicates the SRS ECU has detected excessive resistance in the driver left knee airbag igniter circuit. This is a medium-severity safety issue because the knee airbag may not deploy correctly in a crash. Common causes include a loose or oxidized yellow airbag connector or an internal airbag igniter fault. Visit an authorised dealer for inspection and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1The knee airbag wiring harness connector (yellow plug, usually located behind the driver-side knee trim panel) is loose, has backed-out terminals, or has oxidized pins, causing increased contact resistance.
  • 2Broken spiral resistance wire or poor solder joint in the airbag module internal igniter (component aging or production batch defect)
  • 3Physical damage to the wiring harness: Foot contact abrades the wiring harness in the driver's footwell, or the wiring harness below the steering column rubs against a metal bracket, causing partial breakage of the copper wires.
  • 4Reference drift in the SRS ECU internal sampling circuit or A/D converter fault (less common; cross-check to rule out ECU false reporting).
  • 5Water intrusion into the knee airbag connector after vehicle wading or washing corrodes the pins and forms an oxide layer, resulting in high resistance.
What To Do
  • 1
    Safe power-off: Turn off the ignition switch, 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 water ingress or green oxidation on the terminals.
  • 3
    Resistance measurement: Use a digital multimeter (verify it is in resistance mode with no current output, or use a dedicated airbag resistance measuring tool) to measure the resistance across the airbag inflator terminals. Normal value: 2.0-3.0Ω. If >3.5Ω, the resistance is too high.
  • 4
    Segmented isolation: 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 procedure: If the connector is oxidized, clean it 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: Restore the connection and turn on the power. Use the VDS2000 or a dedicated BYD diagnostic tool to clear the fault code. Execute 'SRS system self-learning' or 'configuration coding'. Perform a road test to verify the fault code does not recur and the airbag indicator light turns off.
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 Atto 8 Fault Codes
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.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.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.