AU
B1780

This fault code indicates the SRS (Supplemental Restraint System) control module failed to detect the normal load resistance (typically 2 — Seal U

Safety System

This fault code indicates the SRS (Supplemental Restraint System) control module failed to detect the normal load resistance (typically 2.0-3.0Ω) in the Second Row Right Seatbelt Pretensioner circuit during the self-check, registering an open circuit.

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

It contains an igniter and a gas generator.

During a frontal or side collision, it detonates to produce high-pressure gas, instantly retracting the seatbelt to firmly secure the occupant in the seat and prevent secondary impact injuries.

This fault completely disables the second-row right seatbelt pretensioner.

The SRS enters degraded mode and continuously illuminates the instrument cluster airbag warning light (SRS light).

Some models may also disable the coordinated deployment of the corresponding side airbag, significantly reducing the occupant protection level in a crash.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Loose connector, backed-out pins, or poor contact at the pretensioner wiring harness connector (usually a yellow plug) under the right middle-row seat, commonly occurring after seat removal and installation, frequent operation of the third-row access function, or failing to fully seat the connector after carpet cleaning.
  • 2Open circuit or abnormal resistance (>5Ω or <1Ω) in the seat belt pretensioner internal igniter. Likely causes include exceeded pretensioner service life, internal moisture ingress, or previous external impact.
  • 3Long-term mechanical chafing of the floor wiring harness within the seat slide rail adjustment range damages insulation and breaks wires, or seat mounting bolts pinch the wiring harness during installation, causing a concealed open circuit.
  • 4Driving through water or a blocked sunroof drain causes water accumulation on the floor. Water ingress and oxidation at the pretensioner connector (located at the lowest point under the seat) triggers an intermittent high-resistance or open-circuit fault.
  • 5During non-professional modifications (such as installing aviation seats, wooden flooring, or 360 soft trim), technicians accidentally cut or short-circuit the pretensioner wiring harness, or fail to use the dedicated shorting plug.
What To Do
  • 1
    Safe power-off and discharge: Turn the ignition switch to OFF, disconnect the negative battery cable, and wait at least 3 minutes (5 minutes for some models) to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Physical connection check: Move the right middle-row seat fully forward and fully rearward. Verify the yellow SRS-specific connector under the seat (usually equipped with a double-locking mechanism) is fully seated. Inspect the terminals for green copper corrosion, backed-out pins, or cracks in the connector housing.
  • 3
    Static resistance measurement: Reconnect the battery (for measurement only). Use a digital multimeter to measure the resistance between the pretensioner connector terminals. Standard value: 2.0 ± 0.5 Ω (refer to the vehicle repair manual). An infinity (OL) reading confirms an open circuit inside the pretensioner or wiring. A 0 Ω reading indicates a possible short circuit.
  • 4
    Harness continuity and insulation test: Disconnect the SRS control module connector. Measure harness continuity between the pretensioner connector and the corresponding ECU pins (< 1 Ω). Measure insulation resistance from the harness to body ground and between the wires (> 1 MΩ). Carefully inspect the harness sleeve at the seat slide rail bend.
  • 5
    Component replacement verification: If circuit tests are normal, replace the pretensioner assembly with a known good unit (Note: Do not use an analog multimeter or apply power to measure pretensioner resistance; use only a digital multimeter for a quick spot check), or swap the left and right pretensioners to confirm if the fault follows the component.
  • 6
    System configuration and reset: After repairing the fault, connect the diagnostic tool (VDS2000 or Launch X431), access the SRS system, and perform "Coding Configuration" (for some models, write specific resistance parameters after replacing the pretensioner). Clear the fault codes, perform the "System Self-check" or "Crash Test" function, and verify the B1780 status changes to "History Fault" or "No Fault". Finally, perform a road test to verify the repair.
Real-World Cases
BYD DTC AI Analysis

Song MAX pretensioner connector loose after seat removal and refitting

✓
Symptoms: After routine maintenance, the SRS warning light on the dashboard stayed on constantly. VDS scan retrieved current DTC B1780. Diagnosis: The right middle-row seat slides frequently due to third-row entry function use. The yellow pretensioner connector locking tab under the seat had not fully engaged, leaving approximately 0.5mm of clearance; driving vibration caused poor contact. Pretensioner body resistance measured 2.3Ω (normal), but the connector terminals showed slight oxidation. Resolution: Cleaned the connector terminals with dedicated terminal cleaner, reseated the plug until the locking tab clicked, and secured the wiring harness to the stationary section of the seat frame using cable ties to prevent interference with the slide rails. Fault resolved.
BYD DTC AI Analysis

Tang DM seatbelt pretensioner internal squib open circuit

✓
Symptoms: Vehicle in normal use with no collision history. SRS light stays on after startup. Diagnostic tool reads B1780; code will not clear. Diagnosis: Disconnected battery negative terminal. Unplugged right middle-row pretensioner connector. Measured pretensioner pins directly: infinite resistance (open). Removed seatbelt assembly (pretensioner integrated with retractor). Found internal igniter solder joint fractured from metal fatigue due to long-term micro-vibration. Solution: Replaced right middle-row seatbelt assembly (includes pretensioner). Part number per EPC: BYD-58191-XXXX. Performed "Seat Belt Pretensioner Resistance Learning" with diagnostic tool. Cleared fault codes. System returned to normal.
BYD DTC AI Analysis

Seat bracket chafed Yuan EV floor wiring harness

✓
Symptoms: SRS light flashes intermittently on rough roads; normal on smooth surfaces. DTC B1780 stored as intermittent fault. Diagnosis: Checked connections under the seat—all secure. Measured pretensioner resistance while sliding the seat back and forth; the reading jumped from 2.1Ω to open circuit. Pulled back the carpet and inspected the floor harness: the front seat mounting bracket’s sharp inner edge had worn through the wiring. Vibration caused the two conductors to intermittently disconnect. Fix: Repaired the damaged conductors. Re-wrapped with waterproof tape and corrugated tubing. Rerouted the harness to avoid the seat slide path. Added a rubber sleeve to the sharp bracket edge. Fault eliminated.
BYD DTC AI Analysis

Qin Pro EV: Aftermarket seat cover installation left connector unplugged

✓
Symptom: Customer reported the SRS warning light lit after having leather seat covers fitted at the dealership. Diagnosis: During the seat cover installation, the technician removed the right middle row seat but failed to reconnect the pretensioner connector. The connector hung loose beneath the seat with the locking mechanism damaged. Resolution: Replaced the damaged SRS connector (with integral shorting bar), re-crimped the terminals and applied waterproofing. Reconnected the harness, cleared the fault codes with a scan tool, performed an SRS cyclic test, and confirmed all airbags and pretensioners functioned normally.
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.