AU
B177E

DTC B177E indicates the SRS (Supplemental Restraint System) detects the resistance of the right middle-row seat belt pretensioner (typically the second-row right seat belt retractor pretensioner) falls below the system threshold (generally below 1 — Seal 6 EV

Safety System

DTC B177E indicates the SRS (Supplemental Restraint System) detects the resistance of the right middle-row seat belt pretensioner (typically the second-row right seat belt retractor pretensioner) falls below the system threshold (generally below 1.0-1.5Ω, depending on vehicle calibration).

As a pyrotechnic actuator in the airbag system, the pretensioner contains an internal resistance heating wire.

Standard resistance typically falls within 2.0±0.5Ω.

Low resistance indicates a short circuit risk in the loop.

Potential causes include an inter-turn short in the internal pretensioner coil, a wiring harness short to ground, or a short between connector terminals.

This fault causes the SRS ECU to register abnormal pretensioner operation.

During a collision, the ECU may inhibit pretensioner deployment, preventing the seat belt from retracting.

The fault also continuously illuminates the airbag warning lamp and triggers a buzzer alarm on certain models.

The B177E fault code on the Seal 6 EV indicates low resistance in the right middle-row seat belt pretensioner circuit, suggesting a possible short. Severity is medium because the airbag warning light may stay on and pretensioner operation could be inhibited in a crash. Common causes include an internal pretensioner short or damaged wiring under the seat. Visit an authorised dealer for SRS diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Pretensioner internal short circuit: Damaged or damp insulation on the pretensioner internal heating coil causes an abnormal drop in resistance (measured value may be <1.0Ω).
  • 2Harness short to ground: Prolonged friction or compression damaged the insulation on the floor harness or the harness under the right middle-row seat, grounding it to the vehicle body metal.
  • 3Connector water ingress and corrosion: Vehicle wading, floor water ingress, or water entering the pretensioner connector (usually located under the seat or below the B-pillar) during a car wash causes a short circuit between terminals or a short to ground.
  • 4SRS ECU detection error: Internal sampling circuit fault in the airbag control module or reference resistor drift, causing a false low resistance reading.
  • 5Improper repair procedures: measuring a powered pretensioner with a low-impedance multimeter, or pinching the wiring harness with the seat rail during previous repairs.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (3 minutes for some models) to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Physical inspection: Check the right middle-row seat belt assembly appearance, under-seat wiring harness routing, and pretensioner connector (usually located at the seat base or inside the side trim panel) for water stains, corrosion, damage, or backed-out terminals.
  • 3
    Pretensioner body measurement: Disconnect the pretensioner connector. Use a high-precision digital multimeter (0.1Ω resolution) to measure the resistance between the two pretensioner terminals. Standard value: 1.5-3.0Ω. If the resistance is <1.0Ω, replace the pretensioner assembly.
  • 4
    Harness insulation check: Measure resistance between the harness side of the pretensioner connector and body ground (should be >1MΩ). Measure resistance between the two harness wires (should be >1MΩ). Locate the short circuit.
  • 5
    ECU-side inspection: Inspect the corresponding SRS ECU connector terminals (refer to the wiring diagram; usually pins 19/20 or marked RMPR/RMPR+) for oxidation or deformation. Measure the wiring harness continuity between the ECU and the pretensioner.
  • 6
    Replacement verification: Connect a standard 2.0Ω resistor or a known-good pretensioner to the circuit. Clear the fault code and check if it returns to isolate the fault to the wiring or the ECU.
  • 7
    System reset: After confirming the fault is resolved, connect the battery. Use the VDS or X-431 diagnostic tool to clear the fault code, perform the SRS system self-check, and verify normal quiescent current and communication.
Real-World Cases
BYD DTC AI Analysis

Song MAX right second-row pretensioner short circuit after water wading

✓
Vehicle wading depth exceeded floor level; SRS warning light stays on. Retrieved DTC B177E. Removed right middle row seat and found water stains on pretensioner connector (inner side of seat rail). Corroded terminals caused a short between positive and negative, measuring 0.6Ω. Cleaned connector with electronic cleaner, dried with compressed air; measured pretensioner body resistance at 2.1Ω (normal). Re-wrapped connector with waterproof tape and adjusted harness position to avoid water pooling in low spots. Fault resolved.
Original source ↗
BYD DTC AI Analysis

Qin Pro EV pretensioner resistance fault after accident repairs

✓
Right rear impact repair. Cluster showed "Check Airbag System." Scan tool retrieved DTC B177E. Right middle row seat belt pretensioner measured 0.8Ω (below specification)—the collision had internally damaged the pretensioner and partially shorted the coil. Replaced the right middle row seat belt assembly (includes pretensioner). Performed configuration coding with the diagnostic tool (programmed VIN and vehicle configuration). Cleared fault codes; system normal.
Original source ↗
BYD DTC AI Analysis

Tang DM Intermittent B177E Troubleshooting

✓
SRS warning light occasionally lit on rough roads; remained off on smooth surfaces. Retrieved historical DTC B177E. Found the right middle row seatbelt pretensioner harness retaining clip loose behind the seat rail. Seat movement rubbed the harness against the rail edge, damaging insulation and causing intermittent short to ground. Repaired the damaged wiring, wrapped it in abrasion-resistant corrugated tubing, and re-secured the routing to clear moving components. Fault has not recurred.
Original source ↗
BYD DTC AI Analysis

Poor contact at e5 pre-tensioner connector caused abnormal resistance.

✓
SRS light stayed on. Scan showed DTC B177E. Found the right middle row seatbelt pretensioner connector not fully locked (locking tab fractured), causing poor terminal contact and abnormal resistance (resistance intermittently read low). Also found slight oxidation inside the connector. Replaced the connector housing, cleaned the terminals, applied conductive protectant, and secured the connection. Circuit resistance measured 2.0Ω. Cleared the fault code; issue resolved.
Original source ↗
BYD DTC AI Analysis

Aftermarket audio installation on Song DM accidentally damaged seatbelt pretensioner wiring harness

✓
The SRS warning light illuminated after the owner installed floor audio equipment. Inspection revealed that during the modification, the right-hand second-row seat mounting bolt pinched the seatbelt pretensioner harness, fracturing the internal copper conductors and shorting them to ground. Resistance measured 0.3Ω. Stripping back the harness showed both wire insulations were damaged. Cut out the damaged section (leaving sufficient length), re-soldered the wires using waterproof heat-shrink tubing, and re-routed the harness to clear the seat mounting points. Fault resolved.
Original source ↗
Other Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.