AU
B174B

This DTC indicates the SRS (Supplemental Restraint System) ECU detects that the igniter circuit resistance for the left rear side airbag (typically located in the left rear seat backrest side or C-pillar trim panel) is below the calibrated threshold (typically < 1 — Atto 8

Safety System

This DTC indicates the SRS (Supplemental Restraint System) ECU detects that the igniter circuit resistance for the left rear side airbag (typically located in the left rear seat backrest side or C-pillar trim panel) is below the calibrated threshold (typically < 1.5Ω; standard value 2.0 ± 0.3Ω).

Electrically, low resistance usually indicates a parallel resistance path in the circuit.

Possible causes include an inter-turn short circuit in the internal igniter coil, damaged wiring harness insulation shorting to body ground, or conductive contamination between connector terminals.

This fault forces the SRS into a degraded mode.

During a collision, the ECU may disable deployment of the affected airbag due to the circuit anomaly (fail-safe).

Furthermore, if the resistance drops near 0Ω, the system risks inadvertent deployment.

This safety-critical fault requires immediate repair.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Internal short circuit in the left rear side airbag squib: Aging, moisture ingress, or manufacturing defects cause an inter-turn short circuit in the gas generator ignition coil inside the airbag module, reducing total resistance.
  • 2Harness chafed and shorted to ground: Damage to the harness sleeve at the seat frame, slide rail, or hinge causes the igniter harness (usually yellow) to contact the vehicle body metal, creating a parallel resistance path to ground.
  • 3Connector water ingress or corrosion: An aging left rear door frame seal or driving through water allows moisture to enter the seat side airbag connector, causing electrolytic corrosion between the terminals or forming a conductive water film, which reduces circuit resistance.
  • 4Rear seat modification damage: During unprofessional installation of seat covers or heating pads, or during seat repairs, fixing screws pierce or pinch the wiring harness, damaging the insulation and causing a short circuit.
  • 5SRS ECU sampling circuit fault: Abnormal internal A/D converter or sampling resistor in the control unit causes a false low resistance report (actual resistance is normal, but the measured value is low).
What To Do
  • 1
    Safety preparation: Set the vehicle to OFF, disconnect the low-voltage battery negative terminal, and wait at least 3 minutes to fully discharge the SRS ECU capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Connect the VDS or dedicated diagnostic tool, read the fault code, confirm B174B is active, and record the freeze frame data (vehicle speed, temperature, etc. at the time of occurrence).
  • 3
    Physical inspection: Inspect the airbag label location on the side of the left rear seat (door side) for external damage, water stains, or signs of removal. Inspect the wiring harness connector under the seat for looseness, backed-out pins, or green corrosion.
  • 4
    Harness isolation test: Disconnect the SRS ECU connector and the airbag module connector. Use a multimeter to measure the resistance to ground of both terminals on the harness side (should be > 1MΩ) and the continuity resistance between the terminals (should be < 1Ω). Verify the harness insulation is good.
  • 5
    Airbag module inspection: Use a dedicated airbag resistance meter (or a digital multimeter on the low-resistance range, confirming equipment safety) to measure the resistance between the two terminals of the airbag module connector. Normal resistance is 2.0±0.3Ω. A measured value < 1.5Ω indicates an internal short circuit in the module.
  • 6
    Fault repair: If the wiring harness has a short circuit, repair the damaged section and protect it with insulating tape and corrugated conduit; if the connector has corrosion, clean it with electrical contact cleaner and apply conductive grease; if the airbag module fails, replace the left rear side airbag module (replace or transfer the wiring harness simultaneously).
  • 7
    System verification: Restore all connections. Reconnect power, use the diagnostic tool to clear the fault code, and perform the SRS system self-check (approximately 20 seconds). Confirm B174B does not reappear. Perform a road test and verify the airbag warning light turns off and remains off.
Real-World Cases
BYD DTC AI Analysis

Poor contact at left rear side airbag connector (BYD Qin)

✓
The airbag warning light on my 2020 BYD Qin stayed on. Using a scan tool, I retrieved DTC B174B-00. I found the left rear seat side airbag connector was not fully seated, with a gap of about 1 mm causing unstable terminal contact resistance that sometimes dropped to 0.8 Ω. I disconnected and reconnected the connector, verified the latch engaged fully, and the resistance returned to normal (2.1 Ω). Clearing the DTC fixed the issue.
Original source ↗
BYD DTC AI Analysis

BYD Tang seat rail wear causes wiring harness short circuit

✓
A 2021 BYD Tang had an intermittent airbag warning light while driving. Diagnostics showed DTCs B174B (resistance too low) and B174C (resistance too high) alternating. Removing and inspecting the left rear seat revealed the seat rail rubbing against the wiring harness during fore-aft adjustment, damaging the yellow airbag harness insulation. The copper wires contacted the metal rail, causing an intermittent short to ground. Replaced the damaged harness section, rerouted and secured the harness, and fitted a protective sleeve. The fault was eliminated.
BYD DTC AI Analysis

BYD Song MAX fault after rear seats removed and refitted

✓
A 2019 Song MAX's airbag warning light came on after the owner removed and refitted the left rear seat cover. Diagnostic testing revealed DTC B174B. Inspection found that during removal, the owner had fully unplugged the side airbag connector on the seat backrest. When reconnecting, they misaligned the pins, which deformed the terminals and caused a short circuit. Replaced the airbag harness connector (with terminals) and restored the connections using a dedicated crimping tool. After clearing the fault code, the system returned to normal.
Other Atto 8 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.