AU
B178112

This fault code indicates the SRS (Supplemental Restraint System) ECU detected an abnormally low-resistance connection between the right second-row seat belt pretensioner igniter circuit and the vehicle power supply (12V constant power or ignition switch power) — Seal U

Safety System

This fault code indicates the SRS (Supplemental Restraint System) ECU detected an abnormally low-resistance connection between the right second-row seat belt pretensioner igniter circuit and the vehicle power supply (12V constant power or ignition switch power).

Under normal conditions, the pretensioner igniter circuit maintains a high-resistance state, consisting only of the 2-3Ω igniter resistance and the wiring harness resistance, and must not have continuity with the power supply line.

A short to power causes the ECU to detect an abnormally high voltage (near battery voltage), triggering the fault code and illuminating the airbag warning lamp.

This fault presents a serious safety risk.

The pretensioner may deploy unintentionally while driving, suddenly tightening the seat belt and injuring the occupant.

Additionally, during an actual collision, circuit protection mechanisms or energy loss through the short circuit may prevent the pretensioner from deploying, failing to protect the occupant.

The B178112 fault code on the Seal U indicates a high-severity SRS fault where the right second-row seat belt pretensioner igniter circuit may be shorted to 12V power. This can trigger the airbag warning light and may cause unintended pretensioner deployment or failure in a crash. Common causes include damaged B-pillar or floor wiring, or water-related connector corrosion. Stop using the seat if possible and visit an authorised dealer. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Right B-pillar or floor wiring harness insulation wear: Long-term vibration, friction, or seat adjustment damages the insulation on the second-row seat belt pretensioner wiring harness (usually located inside the B-pillar trim or under the seat). The harness contacts a 12V body power wire (such as constant power, ignition power, or a lighting circuit), causing a short circuit.
  • 2Connector water ingress and corrosion: Vehicle wading, a blocked sunroof drain, or improper car washing causes water ingress into the right second-row seat belt pretensioner connector (usually located under the seat or at the base of the B-pillar). This causes electrolytic corrosion between the terminals, forming a conductive path that shorts the igniter terminal to the power terminal.
  • 3Internal short circuit in the pretensioner unit: Manufacturing defects, aging, or moisture ingress cause a short circuit between the internal bridgewire and the housing of the pretensioner igniter inside the seat belt retractor, or ignition compound leakage causes internal conductivity.
  • 4Caused by incorrect wiring or modifications: During vehicle modifications (such as adding seat heating, ambient lighting, or a dash cam), technicians incorrectly splice the power wire into the SRS wiring harness, or the seat slide rail crushes and damages an improperly secured harness.
  • 5SRS control unit internal fault: Components such as MOSFETs or capacitors in the ECU internal drive or monitoring circuits break down, causing an internal short circuit between the squib driver terminal and the power supply (relatively uncommon, but requires inspection).
What To Do
  • 1
    Safety Preparation: Disconnect the battery negative terminal and wait at least 3 minutes to fully discharge the SRS capacitor. Wear an anti-static wrist strap. Do not use a standard multimeter resistance setting to measure directly near the airbag system (use a dedicated airbag resistor or diagnostic tool guided function) to prevent accidental pretensioner deployment.
  • 2
    Fault Confirmation and Data Recording: Use the dedicated BYD diagnostic tool (VDS or ED400) to read the complete DTC snapshot data. Record the voltage, resistance values, and environmental data at the time of the fault. Confirm whether B178112 is a current fault (Active) or a history fault (History).
  • 3
    Visual inspection: Remove the right middle-row B-pillar lower trim panel and seat. Inspect the seat belt pretensioner connector (yellow dedicated plug) for looseness, water ingress, corrosion, or burning. Inspect the wiring harness corrugated conduit for damage or crushing.
  • 4
    Circuit insulation and continuity test: Disconnect the right middle-row pretensioner wiring harness from the SRS ECU connector. Use an insulation resistance tester (megohmmeter, 500V range) to measure the insulation resistance between the pretensioner circuit and the power supply. The normal value is >10MΩ. Use a multimeter on the voltage setting to check the wiring harness for abnormal voltage to the power supply.
  • 5
    Pretensioner unit test: Replace the pretensioner with a dedicated airbag resistor (2Ω). Check if the fault code changes to an open circuit fault (e.g., B178111 or high resistance fault) to determine whether the fault is in the wiring harness or the pretensioner. If the fault persists after replacement, the fault is in the wiring harness or ECU. If the fault changes, the pretensioner has an internal short circuit.
  • 6
    Detailed wiring harness inspection: Follow the routing of the middle-row right floor harness (usually along the floor centerline or right door sill) and open the corrugated conduit section by section for inspection. Focus on seat mounting bracket edges, harness retaining clips, and points of contact with the vehicle body metal. Repair damaged insulation. Replace the wiring harness assembly if necessary.
  • 7
    SRS ECU verification: If the wiring harness and pretensioner are normal, measure continuity between the corresponding SRS ECU pin and power. If the ECU has an internal short circuit, replace the SRS control unit, then perform configuration coding and immobilizer matching.
  • 8
    Repair Verification: After repairing or replacing the faulty component, reconnect all connectors and restore battery power. Use the diagnostic tool to clear the fault code, then perform the 'SRS System Self-check' or 'Collision Output Test' (Output Test) to confirm B178112 does not return. Finally, perform a road test to confirm the airbag warning light remains off and the system logs no new fault codes.
Real-World Cases
BYD DTC AI Analysis

Worn right B-pillar wiring harness caused intermittent short circuit

✓
Vehicle: Qin EV 2019. Customer reported the airbag warning light occasionally comes on, particularly after adjusting the middle row seatback angle. VDS scan showed DTC B178112 (active fault). Removed the right middle row B-pillar trim and found the seatbelt pretensioner harness insulation worn through at the retainer clip below the B-pillar, exposing copper and contacting the constant 12V power wire for the rear door speakers. Seat adjustment pulled the harness, causing intermittent shorts. Repaired the damaged section with heat-shrink tubing, re-routed the wiring to clear moving parts, and relocated the mounting point to increase slack. Fault resolved. No recurrence at 1-month follow-up.
BYD DTC AI Analysis

Connector corrosion after water wading caused the fault

✓
Vehicle: BYD E2 2019. Water entered the cabin through a blocked sunroof drain tube and flooded the floor. After the workshop drained the water and dried the interior, the owner found the airbag warning light stayed on. The scan tool showed DTC B178112 and multiple seat occupancy sensor faults. Inspection revealed obvious water stains and green copper corrosion inside the right middle-row seatbelt pretensioner connector (located at the lowest point under the seat). An electrolyte conductive path had formed between the igniter terminal and the adjacent power terminal (which supplies the seat heater, optional on this vehicle). Repair: Replaced the pretensioner wiring harness connector (including the waterproof seal), cleaned the ECU-side connector, and treated the pins with dedicated contact cleaner. The pretensioner itself was undamaged, so we did not replace the retractor assembly.
BYD DTC AI Analysis

Internal short circuit in the pretensioner body caused abnormal resistance.

✓
Vehicle: BYD E3 2020. Accident-repaired vehicle (right rear side collision). Replaced the right second-row seat belt assembly (includes pretensioner). After installation, DTC B178112 appeared and would not clear. Checked new component wiring connections – normal. Measured pretensioner resistance at only 0.5Ω (normal: 2.0–3.0Ω), with 12V to body ground. New part had an internal quality defect: igniter bridge wire shorted to housing. Replaced the pretensioner assembly with a genuine OEM unit (part number must match vehicle configuration; distinguish between versions with and without force limiters). Fault cleared. Use BYD genuine parts when replacing SRS components on accident vehicles. Aftermarket parts risk resistance mismatch or short circuits.
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.