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 — Seal U

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.

The B174B fault code on the Seal U indicates low resistance in the left rear side airbag igniter circuit. This is a medium-severity SRS fault that may stop the airbag deploying correctly, and in rare cases could risk unintended deployment. Common causes include a shorted airbag squib or damaged wiring chafing against the body. The recommended fix is inspection and repair by an authorised dealer. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal short circuit in the left rear side airbag squib: Aging, moisture ingress, or manufacturing defects in the airbag module gas generator ignition coil cause an inter-turn short circuit, reducing total resistance.
  • 2Wiring harness chafed and shorted to ground: A damaged wiring harness sheath at the seat frame, slide rail, or hinge causes the igniter wiring harness (usually yellow) to contact the vehicle body metal, creating a parallel resistance to ground.
  • 3Connector water ingress or corrosion: An aging left rear door frame seal or driving the vehicle through water allows moisture to enter the seat side airbag connector, causing electrolytic corrosion between the terminals or forming a conductive water film that reduces circuit resistance.
  • 4Rear seat modification damage: Unprofessional installation of seat covers or heating pads, or improper seat repairs, causes fixing screws to pierce or pinch the wiring harness, damaging the insulation and causing a short circuit.
  • 5SRS ECU sampling circuit fault: A faulty internal A/D converter or sampling resistor in the control unit causes a false low resistance reading (actual resistance is normal, but the detected 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 for the SRS ECU capacitor to discharge completely to prevent accidental airbag deployment.
  • 2
    Fault confirmation: Connect the VDS or dedicated diagnostic tool. Read the fault code and confirm B174B is active. Record the freeze frame data (vehicle speed, temperature, etc. at the time of occurrence).
  • 3
    Physical inspection: Check the airbag-marked area on the side of the left rear seat (door side) for external damage, water stains, or signs of removal. Check the wiring harness connector under the seat for looseness, backed-out pins, or green corrosion.
  • 4
    Wiring 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 wiring harness side (should be > 1MΩ) and the continuity resistance between the terminals (should be < 1Ω). Verify the wiring harness insulation is good.
  • 5
    Airbag module inspection: Use a dedicated airbag resistance meter (or a digital multimeter set to a low-resistance range, verifying equipment safety first) to measure the resistance between the two terminals of the airbag module connector. The standard resistance is 2.0±0.3Ω. If the measured value is < 1.5Ω, the module has an internal short circuit.
  • 6
    Fault repair: If the wiring harness is shorted, repair the damaged section and protect it with insulating tape and corrugated conduit; if the connector is corroded, clean it with electrical contact cleaner and apply conductive grease; if the airbag module is faulty, replace the left rear side airbag module (replace or transfer the wiring harness simultaneously).
  • 7
    System verification: Restore all connections. Reconnect the power supply, use the diagnostic tool to clear the fault code, and perform the SRS system self-check (approximately 20 seconds). Confirm B174B does not return. 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 Seal U Fault Codes
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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.