AU
B174C1B

This fault code indicates the Airbag Control Module (ACM) detects the squib circuit resistance of the left rear side airbag (typically integrated into the left rear seat backrest side or C-pillar trim panel) exceeds the calibrated upper limit — Atto 8

Safety System

This fault code indicates the Airbag Control Module (ACM) detects the squib circuit resistance of the left rear side airbag (typically integrated into the left rear seat backrest side or C-pillar trim panel) exceeds the calibrated upper limit.

Normal airbag squib resistance ranges from 1.5Ω to 3.0Ω.

The ACM logs a 'high resistance' fault when it detects the value continuously exceeding the threshold (typically above 4.5Ω).

This active safety system functional fault means the airbag may deploy abnormally, deploy late, or fail to deploy completely during a side-impact collision, disabling side protection for the left rear passenger.

An abnormal drop in squib circuit conductivity causes this fault.

Increased contact resistance, a partial open circuit, or an aging internal squib spiral coil can trigger this condition.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Loose, oxidized, or water-corroded airbag wiring harness connector under the left rear seat or at the B-pillar, causing increased contact resistance.
  • 2Frequent seat adjustment causes the wiring harness to chafe at the slide rail mounting point, resulting in partially broken copper strands (poor contact) and a high-resistance condition.
  • 3Aging, open circuit, or manufacturing defect in the left rear side airbag module internal igniter.
  • 4SRS control module (ACM) internal detection circuit fault or terminal pin back-out causing a false abnormal resistance reading.
  • 5Previous water wading, high humidity, or improper washing caused oxidation and formed an oxide layer inside the airbag wiring harness connector.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds to fully discharge the SRS energy storage capacitor. Use the diagnostic tool to read the freeze frame data and confirm B174C1B is a current fault code (Current DTC), not a history fault.
  • 2
    Visual inspection: Remove the left rear seat assembly (or lower C-pillar trim panel), locate the left rear side airbag wiring harness connector (usually a yellow plug), and check for looseness, signs of water ingress, green oxidation, or backed-out pins; check the wiring harness sleeve at the seat slide rail for damage.
  • 3
    Resistance measurement: Use the dedicated SRS low-current resistance tester (do not use a standard multimeter). Disconnect the airbag connector and measure the resistance between the two igniter terminals. The standard value is 2.0±0.3Ω. If the measured value exceeds 4.5Ω or is infinite, the airbag unit is faulty.
  • 4
    Harness continuity check: Use a multimeter in buzzer mode to measure harness continuity between the airbag connector and the SRS control module. Focus on checking the continuity and insulation of the transition harness under the seat. Measure the resistance between both ends of the harness; it should be less than 1Ω.
  • 5
    Substitution test: Connect a 2.0Ω dedicated airbag simulator resistor to the original wiring harness connector. Clear the fault code, power on, and perform a self-check. If the fault code changes to 'B174C-00' (resistance too low) or disappears, the fault is in the airbag unit. If the fault code remains, check the wiring harness or ACM.
  • 6
    Repair procedure: Clean oxidised connectors with electrical contact cleaner and apply conductive grease; repair damaged wiring harnesses by soldering and insulate with double-layer heat-shrink tubing; if the airbag unit is faulty, replace the left rear side airbag assembly (install new retaining bolts simultaneously).
  • 7
    System verification: Restore all connections and connect the battery. Use the diagnostic tool to perform the 'SRS system self-check' and 'crash output test'. Confirm B174C1B no longer appears and the instrument panel SRS warning light turns off normally after the self-check.
Real-World Cases
BYD DTC AI Analysis

Qin Pro DM seat rail wiring harness wear caused resistance drift

✓
A 2019 Qin Pro DM with 32,000 km had an intermittent SRS warning light on the instrument cluster. Retrieved DTC B174C1B (current). Inspection found that when adjusting the left rear seat, the harness beneath the seat chafed against the metal edge of the seat rail, causing one copper wire to break 90% of the way through without fully severing, creating a high-resistance condition (measured at 6.8Ω). Repair: Cut and re-soldered the damaged wire, insulated it with double-layer heat shrink tubing, and repositioned the harness retaining clips to prevent further chafing. Cleared the fault code and monitored for one week; no recurrence.
BYD DTC AI Analysis

Water-wading vehicle airbag connector oxidation fault

✓
2018 BYD Qin Pro (petrol). Water entered the cabin, reaching seat height. Afterwards, the vehicle logged DTC B174C1B. Removed the left rear seat and found water stains and green copper corrosion inside the airbag harness connector (located on the left side of the seat frame). Surface oxidation on the pins increased contact resistance to 5.2 Ω. Repair: cleaned the male and female terminals thoroughly with precision electronic cleaner, lightly sanded the pins with fine sandpaper until metallic luster was restored, applied special conductive grease, and reconnected. Inspected other airbag harnesses on the same side—no abnormalities found. Reassembled the components and the fault was eliminated.
BYD DTC AI Analysis

Open circuit inside left rear side airbag

✓
2019 Qin Pro DM collision repair. After replacing the left rear side airbag, DTC B174C1B set. Measured the new airbag resistance: infinite. Diagnosed an open in the internal igniter, possibly due to improper transport or storage. Installed a second airbag assembly; resistance 2.1 Ω, normal. However, the fault returned after installation. Further inspection found the airbag control module connector had been jarred loose in the accident, causing pin 14 (left rear side airbag circuit) to back out. Repaired the pin; fault resolved.
BYD DTC AI Analysis

False fault - ACM internal detection circuit malfunction

✓
A 2018 BYD Qin Pro had DTC B174C1B, but the left rear side airbag resistance measured 2.0Ω (normal) and the wiring harness showed good continuity with no damage. Tried replacing the airbag assembly; the fault remained. Checked the ACM diagnostic pulse waveform with an oscilloscope and found it abnormal. Diagnosed an internal constant current source circuit fault in the SRS control module. Replaced the SRS control module (requires VIN matching and coding configuration), after which the fault code cleared and the system returned to normal.
BYD DTC AI Analysis

Poor connector contact in low-temperature conditions

✓
A 2019 Qin Pro DM owner in a cold northern region reported the SRS warning light coming on during winter morning startups and going out after driving for some time. The vehicle stored historical fault code B174C1B. Inspection found no oxidation on the airbag connector under the left rear seat, but at low temperatures (-15°C), thermal expansion and contraction created a micro-gap between the plug and socket, increasing contact resistance to 4.2Ω (threshold). Repair: Adjusted the connector locking tab to ensure full engagement, applied conductive grease rated for low-temperature performance to the pin surfaces, and added wiring harness securing points to relieve stress. Follow-up low-temperature testing confirmed the fault did not return.
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.