AU
B1777

This fault code indicates the Airbag Control Module (ACM) detects a 0 ohm circuit resistance in the left middle-row (second-row left) seatbelt pretensioner — Seal U

Safety System

This fault code indicates the Airbag Control Module (ACM) detects a 0 ohm circuit resistance in the left middle-row (second-row left) seatbelt pretensioner.

In a normal SRS system, the pretensioner functions as a pyrotechnic actuator with a standard resistance typically between 1.8Ω and 3.0Ω (depending on the specific vehicle model).

A 0 ohm resistance indicates a hard short in the circuit, usually a short to ground or a short between terminals.

This causes the ACM to flag the pretensioner as failed.

This condition triggers the airbag system fail-safe mechanism, potentially preventing the corresponding airbag and related restraint systems from deploying during a collision.

The instrument cluster SRS warning light remains illuminated, posing a severe safety hazard.

The B1777 fault code on the Seal U indicates a short circuit in the left second-row seatbelt pretensioner, with the Airbag Control Module reading 0 ohms instead of normal resistance. Severity is medium, but it can affect SRS protection and keep the airbag warning light on. Common causes include moisture in the connector or chafed wiring under the seat or B-pillar. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Water ingress or moisture in the pretensioner connector causing a short circuit between terminals, commonly due to vehicle wading, leaking interior cleaning fluid, or aging floor sealing strips.
  • 2Wiring harness under the seat or inside the B-pillar is chafed or worn. Damaged wire insulation contacts the vehicle body metal, causing a short to ground.
  • 3Improper handling during seat removal, installation, or interior modification causes incomplete pretensioner connector engagement, deformed terminals, or a broken connector latch shorting against the bracket.
  • 4Short circuit in the pretensioner unit internal bridge wire (very rare; usually due to a manufacturing defect, abnormal voltage surge, or abnormal internal state following deployment)
  • 5Airbag Control Module (ACM) internal detection circuit fault or damaged sampling resistor causing an erroneous resistance reading of 0 (rule out via cross-checking).
What To Do
  • 1
    Use the BYD VDS2000 or a dedicated diagnostic tool to read fault codes. Confirm B1777 is a current (Active) fault, not a historical fault. Record freeze frame data and check for related fault codes such as B17771A (resistance below threshold).
  • 2
    Disconnect the battery negative terminal and wait at least 3 minutes to fully discharge the SRS system. Inspect the left second-row seat belt pretensioner connector (usually under the left side of the seat or at the base of the B-pillar) for water ingress, green corrosion, loose connections, or deformed terminals.
  • 3
    Disconnect the pretensioner connector and use a digital multimeter to measure the resistance between the two terminals of the pretensioner unit. The normal value is 1.8-3.0 Ω (refer to the specific vehicle workshop manual). If the resistance is close to 0 Ω, replace the pretensioner assembly (this usually requires replacing the seat belt as well).
  • 4
    Keep the pretensioner connector disconnected. Measure the resistance between both terminals of the harness-side connector and body ground (GND). The resistance must be infinite (OL). If the resistance is 0 or close to 0, inspect the yellow wiring harness (dedicated SRS harness color) for damage. Focus on the seat slide rail interference area and the floor harness pass-through grommet.
  • 5
    Measure the resistance between the two terminals on the harness-side connector. The resistance must be infinite. Continuity indicates a short circuit between the positive and negative harness wires. Strip back the harness sheath to locate and repair the damaged point. Replace the entire wiring harness if necessary.
  • 6
    After repairing or replacing damaged components, apply double insulation using insulating tape or heat-shrink tubing. Reconnect the connector, ensuring the locking tab engages fully. Reinstall the battery, clear the fault code, and perform the SRS system self-diagnostic cycle test (cycle the ignition switch ON-OFF 3 times). Confirm DTC B1777 does not return and the SRS warning lamp turns off.
Real-World Cases
BYD DTC AI Analysis

Water ingress in connector caused short circuit in flood-damaged vehicle

✓
A 2020 BYD Song MAX drove through floodwater to hub depth after heavy rain. The SRS warning light remained on at startup the next day. VDS2000 showed active fault code B1777. Removed the left middle-row seat trim panel and found standing water in the pretensioner connector. Copper terminals showed black oxidation and verdigris, causing a short between terminals. Cleaned the connector with electronic cleaner, blew it dry with compressed air, and measured normal pretensioner resistance at 2.1Ω. Reconnected the plug, applied insulating silicone grease, and wrapped for waterproofing. Fault cleared.
BYD DTC AI Analysis

Seat modification pinched and chafed the wiring harness.

✓
The owner of a 2019 Yuan EV retrofitted a seat ventilation system to the left middle row seat. During installation, the owner did not secure the loom with dedicated wiring harness clips, pinching the yellow pretensioner wiring against the edge of the seat rail bracket. As the seat moved back and forth, the insulation wore through and the internal conductors shorted to earth. Pretensioner body resistance measured normal, but the harness side read 0.3Ω to earth. Rerouted the wiring to clear the seat rail travel path, protected the loom with abrasion-resistant corrugated tubing, repaired the damaged conductors and resolved the fault.
BYD DTC AI Analysis

Connector contact fault after accident repair

✓
Following a minor collision on the left rear side, this 2021 Tang had the left rear door and B-pillar trim replaced at the dealership. One week after delivery, the customer reported the SRS warning light occasionally illuminating. Diagnostics showed intermittent B1777. Inspection revealed the pretensioner connector was not fully seated during reinstallation of the lower B-pillar trim, and the connector locking tab had broken. This let the connector contact the body metal bracket during driving vibrations, causing an intermittent short. Replaced the connector housing, re-crimped the terminals, and fully seated the connection. Fault has not recurred.
BYD DTC AI Analysis

Pretensioner internal short circuit failure

✓
2019 BYD Yuan, 80,000 km, no accident history. SRS warning light suddenly illuminated. Read current DTC B1777; clearing failed. Disconnected the connector and measured resistance across the pretensioner terminals: 0.1 Ω (near short). Diagnosed as internal igniter bridge wire insulation breakdown or moisture in the pyrotechnic element causing conductivity. Replaced the left middle row seatbelt pretensioner assembly (including seatbelt retractor). Performed Coding and airbag system calibration with the diagnostic tool. Fault resolved.
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.