AU
B177D1A

DTC B177D1A indicates the SRS (Supplemental Restraint System) control unit detects the right middle-row seat belt pretensioner circuit resistance is abnormally low, approaching 0 ohms (standard value is typically 2 — Seal U

Safety System

DTC B177D1A indicates the SRS (Supplemental Restraint System) control unit detects the right middle-row seat belt pretensioner circuit resistance is abnormally low, approaching 0 ohms (standard value is typically 2.0–5.0 Ω).

This indicates a short to ground, a short between wires, or an internal short circuit in the pretensioner squib circuit.

This fault causes the SRS to enter a degraded protection mode.

During a collision, the right middle-row seat belt pretensioner may fail to deploy and retract.

The fault also illuminates the Airbag Warning Light continuously, compromising occupant passive safety protection.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Damaged insulation on the pretensioner wiring harness under the right middle-row seat or near the B-pillar causes the wire to contact metal body parts, creating a short to ground.
  • 2Water ingress, moisture, or corrosion in the seat belt pretensioner connector (usually located under the seat rail or inside the B-pillar trim) causing a short circuit between terminals or a short to ground.
  • 3The pretensioner internal squib short-circuits due to a manufacturing defect or aging, dropping the resistance to 0 Ω.
  • 4Water enters the SRS wiring harness connector during vehicle wading or interior cleaning, causing a temporary or continuous short circuit.
  • 5Frequent fore-and-aft seat adjustment or seat modification causes mechanical interference between the wiring harness and the seat frame. Prolonged chafing damages the wiring harness and causes a short circuit.
What To Do
  • 1
    Perform the high-voltage safety procedure: disconnect the low-voltage battery negative terminal and wait at least 3 minutes to ensure the SRS backup power supply discharges fully, preventing accidental airbag deployment.
  • 2
    Locate the right middle-row seat belt pretensioner: Remove the right middle-row seat (or check below the B-pillar) and find the pretensioner assembly and its 2-pin connector (usually a yellow waterproof connector).
  • 3
    Visual inspection: Check the connector for water ingress, terminal corrosion, burn marks, or physical damage. Check the wiring harness for wear, pinching, or breakage.
  • 4
    Resistance measurement (disconnected): Use a multimeter to measure the resistance between the pretensioner body terminals. Normal resistance is 2.0-5.0Ω. If the resistance is 0Ω, replace the pretensioner assembly.
  • 5
    Harness continuity check: Measure resistance to ground at the harness-side connector (should be infinite) and measure continuity between both ends of the harness. If continuity to ground exists, inspect the harness section by section to locate the damage.
  • 6
    Insulation repair or replacement: If the wiring harness has damage, repair it using heat-shrink tubing or waterproof tape; if the connector has damage, replace the connector; if the wiring harness has extensive damage, replace the entire SRS wiring harness.
  • 7
    System reset and verification: Reconnect all connectors, reconnect the battery, use VDS or a dedicated diagnostic tool to clear the fault code, perform the SRS system self-check procedure, and verify B177D1A does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Water-damaged vehicle: Right middle row pretensioner circuit short

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Symptoms: A 2020 BYD E3 developed a constant airbag warning light after wading through deep water in heavy rain. VDS read DTC B177D1A. Diagnosis: Removed the right second-row seat. Found obvious water stains in the SRS harness connector beneath the seat, with terminals oxidized. Measured 0.2 ohms between the pretensioner harness side and ground, confirming a short to ground. Harness insulation had worn through at the seat rail mounting clip; water entered during wading and caused the short. Repair: Cleaned the connector terminals with anhydrous alcohol, thoroughly dried with a heat gun, repaired the damaged harness insulation, rewrapped with waterproof tape, and rerouted the harness to avoid contact with the seat frame. Cleared the fault codes; system returned to normal.
BYD DTC AI Analysis

Seat modification damaged the pretensioner wiring harness

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Symptoms: The 2019 Qin EV displayed DTC B177D1A and the airbag warning light illuminated after the owner installed a seat cover on the right-hand middle row seat. Diagnosis: Inspection revealed the seat cover mounting clip interfered with the pretensioner wiring harness inside the seat backrest. After removing the seat, we found the clip screw had pierced the harness, causing both positive and negative wires to contact the seat metal frame and short. We measured the pretensioner resistance at 3.2Ω (normal), but the harness side showed 0Ω to ground. Solution: We repaired the damaged harness, replaced the broken wires, rerouted the wiring with corrugated tubing protection, repositioned the seat cover mount to clear the harness routing, and cleared the fault.
BYD DTC AI Analysis

Water ingress corroded the connector, causing an intermittent short circuit

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Symptoms: The 2019 E2 airbag warning light came on intermittently during the rainy season, with DTC B177D1A appearing sporadically. Diagnosis: The right middle row seatbelt pretensioner connector seal had degraded, with green corrosion on the terminals. Resistance read 0Ω in damp conditions but returned to normal when dry. Moisture entered through the failed seal, causing a short circuit at specific humidity levels. Repair: Installed a new waterproof connector (complete with seal and terminals) and waterproofed the wiring harness. Fault eliminated.
Other Seal U Fault Codes
B1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.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.
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.