AU
B177C

DTC B177C indicates a short to ground in the left middle-row seat belt pretensioner ignition circuit — Seal U

Safety System

DTC B177C indicates a short to ground in the left middle-row seat belt pretensioner ignition circuit.

In the BYD SRS (Supplemental Restraint System) architecture, the pretensioner operates as a pyrotechnic device.

Its ignition circuit normally maintains a high-impedance state (normal resistance approx. 2-3 Ω).

The ECU determines a short to ground when it detects circuit voltage continuously below the threshold (<0.5 V) or an abnormal resistance drop (<1 Ω).

This fault causes: 1) left middle-row pretensioner failure, preventing normal deployment to tighten the seat belt during a collision; 2) the SRS to enter degraded mode, potentially affecting normal airbag deployment logic; 3) the ECU to cut circuit power to prevent accidental deployment from short-circuit currents.

This constitutes a severe active safety system fault.

Triggering this fault illuminates the airbag warning light and activates a continuous buzzer alarm.

The B177C fault code on the Seal U indicates a short to ground in the left middle-row seat belt pretensioner ignition circuit. This is a high-severity SRS fault that can stop the pretensioner working and may affect airbag system logic. Common causes include worn under-seat wiring near the seat rail or moisture/corrosion in the connector. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Mechanical wear of under-seat wiring harness: Frequent fore/aft sliding or folding of the left middle-row seat wears through the insulation of the pretensioner wiring harness (usually located near the seat rail). The exposed wire contacts the metal frame, creating a short to ground.
  • 2Connector water ingress and corrosion: During vehicle wading or interior cleaning, a poorly sealed pretensioner connector (located below the B-pillar or under the seat) allows moisture to enter, reducing insulation resistance between terminals or to ground.
  • 3Internal short circuit in the pretensioner body: Manufacturing defects, aging, or previous external impact causes the internal coil of the pretensioner squib to short to the housing.
  • 4Improper accident repair: Failing to properly wrap insulating tape when replacing the pretensioner or repairing the wiring harness after a collision causes the wire to contact the body ground.
  • 5Seat modification damage: Retaining screws pierce the pretensioner wiring harness and cause a short to ground during customer installation of seat covers, heating pads, or a power adjustment mechanism.
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (to ensure the SRS capacitor discharges completely). Wear an anti-static wrist strap. Never directly measure the pretensioner connector using a multimeter resistance setting (use only a dedicated SRS resistance measuring tool or the diagnostic tool active test function).
  • 2
    Visual inspection: Inspect the wiring harness sleeves in the left second-row seat rail area, inside the lower B-pillar trim panel, and under the seat for damage, crushing, water stains, or burn marks. Focus on the wiring harness routing at the seat mounting bolts.
  • 3
    Disconnect the pretensioner: Unplug the left middle row pretensioner connector (usually a yellow plug with a double-locking mechanism). Use the diagnostic tool to read the fault code status. If the fault changes to an open circuit (B177D), the short circuit is in the pretensioner assembly. If the fault remains a short to ground, the short circuit is in the wiring harness.
  • 4
    Harness continuity test: Use a multimeter to measure continuity between both terminals of the pretensioner harness connector and vehicle body ground. The normal reading is infinity. If continuity exists, strip the corrugated conduit section by section along the wiring to check for damage. Focus on the seat hinge movement area.
  • 5
    Pretensioner resistance measurement: Use the dedicated SRS test resistor (2Ω substitute resistor) or the diagnostic tool actuator test function to verify the pretensioner body resistance is within the standard range (1.5-3.0Ω). If out of range, replace the pretensioner assembly.
  • 6
    Insulation repair: Upon finding damage to the wiring harness, wrap the damaged area with double-layer heat-shrink tubing or high-temperature insulating tape. Reroute the harness to avoid interference with moving parts. Install a wiring harness sleeve or cable ties if necessary.
  • 7
    Connector handling: If water enters the connector, clean the terminals with electrical contact cleaner, apply conductive grease, replace the sealing ring, and verify the locking tab is intact.
  • 8
    System reset: After repair, reconnect all connectors and restore the battery. Use VDS or Launch X431 to enter the SRS system and execute "Configuration and Settings" → "Seat Belt Pretensioner Check". Clear the fault code and perform an ignition circuit self-check. Confirm the fault code changes to "History Fault" and the warning light turns off.
  • 9
    Function check: Perform a simulated crash test (use the diagnostic tool to execute the 'Crash Output Test' function; this checks circuit continuity only and does not deploy the pretensioner). Confirm the left second-row pretensioner circuit resistance and voltage parameters are within the normal range.
Real-World Cases
BYD DTC AI Analysis

Song MAX left second-row seat adjustment caused wiring harness wear and short circuit

✓
A 2019 BYD Song MAX with 32,000 km displayed "Check SRS System" on the dash. Scan revealed DTC B177C. Inspection found the wiring harness sheath torn behind the left middle-row seat rail, with two internal wires chafed through from frequent seat adjustment; one had shorted to the seat frame. Passengers regularly adjusted this seat through its full travel, causing excessive flexing at this point. Cut out the damaged section, soldered in extension wires, rerouted the harness to the outside of the seat rail clear of the movement path, and fitted a nylon protective sleeve. The fault has not recurred during three months of monitoring.
BYD DTC AI Analysis

Tang DM pretensioner connector corroded and shorted after driving through water

✓
2021 BYD Tang DM encountered heavy rain and drove through water deep enough to reach the door sills. The following day, the airbag warning light stayed on constantly. The scan tool displayed alternating codes B177C and B177B (short to power). Removed the left middle row seat and found water inside the pretensioner connector at the seat base crossmember; the terminals had oxidised and turned green. The seal ring had deteriorated, allowing water to enter under pressure during wading. Replaced the entire wiring harness plug including the pins, ultrasonic-cleaned the pretensioner socket with alcohol, sprayed WD-40 precision electrical contact cleaner, and applied waterproof silicone grease during reassembly. Also inspected the airbag connector on the same side with no faults found.
BYD DTC AI Analysis

Yuan EV pretensioner improperly installed after accident repair

✓
2019 Yuan EV. Left rear collision. Third-party shop replaced the left second-row pretensioner. One week post-repair, B177C set. Inspection showed the new pretensioner harness had pinched against a retaining clip where it passes through the B-pillar trim, damaging the insulation and shorting to earth. Also found: pretensioner mounting bolts torqued to only 15 N·m (spec: 35 N·m). Root cause: the unauthorised repairer skipped the workshop manual and omitted the dedicated harness clips. Fix: Replaced the damaged harness section (floor harness connector to pretensioner), retorqued the pretensioner to spec, and installed genuine clips to ensure harness routing complies with R&L (wiring diagram).
BYD DTC AI Analysis

Qin Pro heated seat modification inadvertently damaged pretensioner circuit

✓
2020 BYD Qin (petrol). Owner self-installed seat heating pads. While drilling into the left middle-row seat foam to secure the heating element, the installer penetrated the underside wiring harness with the drill bit, simultaneously damaging both pretensioner circuit wires and causing them to contact the seat metal frame. This triggered DTCs B177C and B177A (left middle-row pretensioner open circuit). Measured harness-to-ground resistance: 0.8Ω. Repair: Repaired the damaged harness (replaced the entire floor-side harness connector as the damage was too close to the connector), sealed splices with BYD-specific adhesive-lined heat-shrink tubing, and rerouted heating-pad wiring to the opposite side of the seat, away from the SRS harness. Reminded the owner to keep all modifications clear of airbag and pretensioner harness routing areas.
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.