AU
B177A

DTC B177A indicates the SRS (Supplemental Restraint System) control unit fails to detect the left middle-row (typically left second-row) seat belt pretensioner during self-diagnosis — Seal U

Safety System

DTC B177A indicates the SRS (Supplemental Restraint System) control unit fails to detect the left middle-row (typically left second-row) seat belt pretensioner during self-diagnosis.

This is an open-circuit fault; the ECU detects infinite pretensioner circuit resistance or resistance outside the normal threshold (standard value: 1.5-2.5Ω).

The pretensioner is a key actuator in the passive safety system.

During a frontal collision, a pyrotechnic igniter instantly tightens the seat belt to eliminate slack between the occupant and the belt.

This fault indicates an open pretensioner circuit, preventing pretensioner deployment for this seat during a collision.

This condition may also disrupt the coordinated deployment logic of the same-side or related airbags. "Not present" in this fault code does not indicate a missing physical component; it designates an electrical "communication loss" or "open circuit".

The B177A fault code on the Seal U indicates an open circuit in the left middle-row seat belt pretensioner, so the SRS control unit cannot detect it during self-checks. Severity is medium because this pretensioner may not deploy in a crash and could affect related airbag logic. Common causes include a loose under-seat connector or damaged wiring near the seat track. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Loose pretensioner connector or poor contact: Frequent sliding or folding of the left middle-row seat can loosen the pretensioner wiring harness connector located under the seat or below the B-pillar. On MPV models like the Song MAX, the wide movement range of the middle-row seat subjects the connector to mechanical stress.
  • 2Wiring harness breakage or wear: Long-term bending of the pretensioner wiring harness near the seat track causes fatigue fractures in the internal copper core, or the metal seat frame edge chafes through the harness insulation, causing an open circuit. Common in 7-seat versions of SUV models such as the Tang and Song.
  • 3Internal pretensioner fault: Open circuit in the pretensioner internal igniter or bridge wire. Causes include manufacturing defects, prolonged vibration, or failure to replace the unit after deployment in a previous accident.
  • 4SRS control unit false report: An airbag ECU internal detection circuit fault or software calibration issue causes the system to incorrectly identify the pretensioner as missing. This commonly occurs in specific software versions of early Yuan EV or Qin models.
  • 5Incorrect wiring after accident repair: After a collision or interior removal and installation, the technician incorrectly connected the left second-row pretensioner connector or used a mismatched resistor substitute to bypass the fault, resulting in abnormal system detection.
What To Do
  • 1
    Safety Preparation and Initial Diagnosis: Disconnect the negative battery terminal and wait at least 3 minutes to fully discharge the SRS capacitor. Use a dedicated BYD diagnostic tool (VDS2000 or Launch X431) to read all fault codes. Determine if B177A is an Active or History fault, and record freeze frame data to verify vehicle status at the time of the fault.
  • 2
    Visual inspection and connector check: Check the left second-row seat belt assembly for signs of impact or disassembly. Locate the pretensioner connector (usually on the lower inner side of the seat or inside the B-pillar trim). Verify the connector locks fully and inspect the terminals for oxidation, backed-out pins, or corrosion. Clean with electrical contact cleaner and reconnect if necessary.
  • 3
    Circuit measurement and resistance test: Disconnect the pretensioner connector. Use a multimeter to measure the resistance across the two terminals on the pretensioner-side connector. The standard value is 1.5-2.5 Ω. If the reading is infinite (OL), replace the pretensioner assembly. If the resistance is normal, measure continuity between the harness-side connector and the SRS control unit to check for an open circuit or short to ground.
  • 4
    Wiring harness repair and insulation: If the wiring harness is broken at any point (especially at the seat slide rail bend), strip the harness sheath, solder the break, and insulate the repair using double-layer heat-shrink tubing or waterproof tape. If the wiring harness is severely worn, replace the entire pretensioner wiring harness assembly to prevent recurrence.
  • 5
    Component Replacement and System Calibration: If confirming a faulty pretensioner, replace the left second-row seat belt assembly (including the pretensioner). After installing the new part, use the diagnostic tool to perform 'SRS System Configuration' or 'ECU Coding' (some models require writing the vehicle VIN and configuration code). Next, clear the fault codes and perform a system self-check (usually requiring 3 ignition cycles). Finally, perform an on-vehicle crash simulation test (use a dedicated dummy resistor to verify normal system communication).
Real-World Cases
BYD DTC AI Analysis

Song MAX left second-row seat rail worn through pretensioner wiring harness

✓
A 2019 BYD Song MAX DM with 42,000 km had the airbag warning light illuminated. Scanning revealed code B177A. Removing the left middle seat revealed the pre-tensioner wiring harness had chafed against the metal edge at the front of the seat rail, wearing through the insulation and completely severing all three copper cores. Replaced the pre-tensioner wiring harness assembly (part number: SA-585XXXX), added a rubber protective sleeve and cable ties at the contact point, and rerouted the wiring to prevent future abrasion. Cleared the fault codes and the system returned to normal.
BYD DTC AI Analysis

Tang DM pretensioner connector not connected after accident repair

✓
2021 BYD Tang DM. Following right-front collision repairs at a non-authorised workshop, the instrument cluster displayed a constant airbag warning light. Diagnosis revealed DTC B177A and a right-front impact sensor fault. Inspection found the left second-row seat had been removed for painting during the accident repair. Upon reinstallation, the technician had not fully seated the pretensioner connector, leaving it merely hanging on the clip and causing a loose connection. Reconnected the connector and confirmed the locking tab engaged. Performed 'Crash History Clear' and 'SRS System Reset' using a diagnostic tool. Fault resolved.
BYD DTC AI Analysis

Aftermarket second-row seat modification on Yuan EV dislodged the connector

✓
2019 BYD Yuan EV535. Owner self-installed a heating pad on the left middle row seat but didn't properly refit the trim panels afterward. Seat adjustment pulled the wiring, causing the pretensioner connector to come completely loose. Airbag warning light came on intermittently, particularly on rough roads. Found the connector partially detached with slight terminal oxidation. Fix: Cleaned the terminals, applied conductive grease, re-secured the connector and used cable ties to take up harness slack so seat movement wouldn't pull on the wires.
BYD DTC AI Analysis

BYD Song MAX Pretensioner Internal Open Circuit Replacement Case

✓
A 2019 Song MAX petrol with no accident history. The airbag warning light illuminated suddenly. Resistance measured infinite at the left second-row pre-tensioner connector. Disconnected the pre-tensioner; the body alone still measured infinite resistance, confirming the internal bridge wire had fractured. Replaced the genuine seatbelt assembly (including pre-tensioner, part number: SA-582XXXX). Fault cleared. Analysis suggests internal propellant moisture or long-term micro-vibration caused bridge wire fatigue. Sporadic component failure.
BYD DTC AI Analysis

Qin Pro SRS Control Unit Software Upgrade for False Alarm

✓
2020 Qin Pro developed intermittent B177A faults. Checked wiring harness and pretensioner resistance – both normal. Connectors normal. Consulted BYD TSB: confirmed early-batch SRS control unit (ACU) software calibration issue causes false middle-row pretensioner missing detection during low-temperature cold starts. Action: Upgraded ACU software to V3.2.1 or higher using VDS. No further faults after update.
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.