AU
B177B

DTC B177B indicates the control circuit for the second-row left seatbelt pretensioner has shorted to the vehicle power supply positive (B+) — Seal U

Safety System

DTC B177B indicates the control circuit for the second-row left seatbelt pretensioner has shorted to the vehicle power supply positive (B+).

In the BYD SRS system, the pretensioner operates as a pyrotechnic actuator.

Under normal conditions, the SRS ECU controls the pretensioner via a low-side drive and, during a collision, supplies an instant high current (approximately 2-3A) to ignite the gas generator.

A short to power means the circuit continuously carries an abnormally high potential (12V).

This may result in the following: 1) The SRS ECU falsely detects a permanent trigger state or wiring fault, activating the system protection mechanism. 2) The SRS ECU disables the entire airbag system, preventing protection during a collision. 3) The pretensioner accidentally deploys or the ECU internal driver circuit burns out in extreme cases.

This is a hard fault and requires immediate repair.

The B177B fault code on the Seal U indicates a high-severity fault where the second-row left seatbelt pretensioner control circuit is shorted to battery positive voltage. This can disable the SRS/airbag system and may prevent protection in a crash. Common causes include damaged under-seat wiring from seat movement or water ingress and corrosion in the pretensioner connector. Visit an authorised dealer immediately for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Mechanical wear of the under-seat wiring harness: During fore-aft adjustment of the left middle-row seat (common on 7-seat models such as Song MAX and Tang), the pretensioner wiring harness rubs against the seat slide rail or metal bracket. Prolonged friction damages the insulation and causes a short circuit to the seat power wire (constant live).
  • 2Connector water ingress and corrosion: Water enters the pretensioner connector (typically a yellow waterproof plug) located under the seat or on the B-pillar during vehicle wading, sunroof leaks, or deep interior cleaning. This creates an electrolytic conductive path between the power terminal and the signal terminal.
  • 3Pretensioner internal short circuit: Insulation breakdown between the gas generator igniter bridge wire and the metal housing, or moisture in the igniter charge holder causing ionic conduction (common in high-temperature, high-humidity environments or older vehicles).
  • 4Modification or repair damage: During seat heating/ventilation retrofits, carpet replacement, or accident repairs, fixing screws pierce the wiring harness, or the seat frame and vehicle body pinch the wiring harness, damaging the insulation.
  • 5Internal SRS ECU fault: Power MOSFET breakdown in the pretensioner drive circuit or a shorted freewheeling diode causes a continuous high-level output (rare, usually accompanied by multiple pretensioner fault codes).
What To Do
  • 1
    Safety preparation: Turn off the ignition, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS energy storage capacitor and prevent accidental deployment.
  • 2
    Fault Confirmation: Use the BYD VDS or Launch X-431 diagnostic tool to read fault codes. Confirm B177B is a Current DTC, not a History DTC. Record freeze frame data to observe the vehicle status when the fault occurred.
  • 3
    Visual inspection: Remove the left middle-row seat (usually requires removing 4 retaining bolts). Inspect the pretensioner wiring harness (wrapped in yellow corrugated conduit) under the seat, below the B-pillar, and inside the floor wiring channel for abrasion, cuts, burn damage, or water ingress. Focus on interference points between the wiring harness and the seat slide rail.
  • 4
    Pretensioner unit inspection: Disconnect the pretensioner connector (usually under the seat). Use a multimeter to measure the resistance between the two pins of the pretensioner unit. Standard value: 2.0–3.0 Ω (at 20°C). If the resistance is 0 Ω or infinite, replace the pretensioner assembly. (Note: Do not use the multimeter resistance setting to measure a wiring harness connected to the ECU.)
  • 5
    Harness short to power check: Turn the ignition switch to ON (do not start the engine). Measure the voltage between the two pins of the pretensioner harness-side (ECU side) connector and body ground. Normal voltage is 0V or close to 0V (<0.1V). A 12V battery voltage reading indicates the harness is shorted to power.
  • 6
    Continuity test: Disconnect the SRS ECU connector (located under the centre console or inside the armrest box). Use a multimeter to measure continuity between the pretensioner wiring harness (ECU side to pretensioner side) and a known power wire (such as the seat heater power supply or a constant-power fuse). If continuity exists, locate the short circuit.
  • 7
    Insulation resistance test: Measure the insulation resistance between the two pretensioner harness wires and body ground. The resistance must be greater than 1MΩ. If the insulation is poor, strip back the corrugated conduit in sections to locate the damaged point.
  • 8
    Replacement verification: Replace the original pretensioner with a dedicated 2 Ω resistor (or a known-good pretensioner). Clear the fault code and cycle the ignition switch 3 times. If the fault disappears, this confirms an internal short circuit in the pretensioner. Replace with an OEM pretensioner (replace the seat belt assembly simultaneously, as the pretensioner is usually integrated with the seat belt retractor).
  • 9
    Wiring harness repair: If the wiring harness is damaged, cut out the damaged section. Repair the harness using high-temperature insulating tape (the pretensioner circuit carries high current and ordinary PVC tape melts easily) or heat-shrink tubing. When rerouting the harness, maintain a clearance of more than 20mm from the seat slide rail and secure it with cable ties.
  • 10
    System verification: Reconnect all connectors and the battery. Turn the ignition switch ON and confirm the SRS warning light on the instrument cluster turns off after 6 seconds. Use the diagnostic tool to perform the 'Pretensioner Function Test' (Trigger Test) to verify ECU drive capability. Finally, perform a road test for verification.
Real-World Cases
BYD DTC AI Analysis

Worn seat adjustment rails chafed the wiring harness on the Song MAX, causing a short circuit.

✓
A 2019 Song MAX DM, 42,000 km, SRS warning light constantly on. Read DTC B177B (active fault). Found the left middle seat pre-tensioner harness (yellow) chafing against the seat slide rail bracket underneath. The insulation had worn through, exposing copper that shorted to the seat heater power wire (red, constant live). Cause: Long-term full-load use with frequent fore-aft seat adjustment shifted the harness. Repair: Cut out the worn section and re-crimped terminals. Rerouted the harness outside the slide rail, added a rubber sleeve and secured with cable ties to maintain 30 mm clearance from moving parts. Replaced the pre-tensioner assembly (potential internal damage from the short circuit).
BYD DTC AI Analysis

Tang EV connectors corroded after water ingress from wading

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2021 Tang EV. After driving through standing water during heavy rain, the instrument cluster displayed "Check Airbag System" with DTC B177B. Teardown revealed visible water inside the pretensioner connector (yellow 2-pin plug) beneath the left middle seat; terminals were oxidised and blackened. Cause: Wading depth exceeded floor height. Water entered the connector through the wiring harness corrugated tube; electrolytes created a conductive path between the power and signal pins. Fix: Cleaned the terminals thoroughly with electronic contact cleaner, blew them dry with compressed air (do not use a heat gun—risk of plastic deformation), and applied conductive anti-corrosion grease. Checked the harness seal integrity and replaced the connector assembly if necessary. Cleared the DTC and monitored for one week with no recurrence.
BYD DTC AI Analysis

Yuan EV short circuit caused by bent pins after accident repair

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2019 BYD Yuan EV. Rear-ended on the left rear side and repaired at an external workshop. After the shop replaced the left rear seatbelt assembly (including pretensioner), the instrument cluster lit the SRS warning light and set DTC B177B. Found bent pins on the new pretensioner harness connector from the repair work. The power supply pin (normally fed by the ECU, but contacting body power during connector handling) shorted to an adjacent constant-power wire (such as the seatbelt warning indicator supply). Fix: Straightened the bent pins, confirmed no fractures at the pin roots, and used a dedicated terminal removal tool to adjust the terminal positions. Verified normal pretensioner resistance and harness insulation, then reconnected the harness. After accident repairs, always perform a complete SRS system diagnosis rather than simply clearing fault codes.
BYD DTC AI Analysis

Qin Pro: Aftermarket aviation seat install damaged wiring harness

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2020 BYD Qin Pro (petrol). Owner installed an aftermarket aviation-style seat with leg rest and heating in the left mid-row position. DTC B177B appeared the day after installation. Inspection found self-tapping screws for the seat mounting bracket had penetrated the floor wiring harness, piercing the pre-tensioner supply wire (black/yellow) and seat heating supply wire (red). Cause: No wiring harness probe used; blind drilling. Fix: Removed the aftermarket seat, replaced the damaged pre-tensioner wiring harness assembly (repair not recommended—multiple conductors damaged), and relocated seat mounting points to avoid the harness routing. Owner warned: SRS system wiring harnesses must never be modified or spliced.
BYD DTC AI Analysis

Intermittent short circuit causing sporadic warning light illumination

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A 2019 Song MAX came in with an intermittent SRS warning light that worsened on rough roads. The scanner showed B177B as an intermittent fault. Static harness tests checked out normal, but shaking the left middle-row seat reproduced the fault. Close inspection found the pretensioner harness under the B-pillar was cut by sharp burrs where it passes through the body sheet metal hole. Vehicle vibration caused the damaged insulation to touch body power lines intermittently. Repair: Rebuilt the insulation with double-layer heat shrink tubing. Installed a rubber grommet on the sheet metal hole and eliminated the burrs. Adjusted the harness clip positions to keep the wiring slack throughout the full range of seat movement.
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.