AU
B17801B

DTC B17801B indicates the SRS (Supplemental Restraint System) control unit detects an open circuit in the right middle-row seat belt pretensioner circuit (resistance exceeds the normal range, typically >10Ω or infinite) — Seal U

Safety System

DTC B17801B indicates the SRS (Supplemental Restraint System) control unit detects an open circuit in the right middle-row seat belt pretensioner circuit (resistance exceeds the normal range, typically >10Ω or infinite).

This fault essentially constitutes a 'passenger restraint system communication loss'.

During a collision, the pretensioner for this seat cannot receive the ignition signal and fails to retract the seat belt at the moment of impact to eliminate slack between the occupant and the belt.

This fault triggers the SRS downgrade protection strategy, potentially altering the airbag deployment logic for this seat and surrounding areas (such as delayed deployment or non-deployment), severely compromising passive safety performance.

For BYD E2/E3/Qin EV models, the right middle row typically refers to the second-row right seat.

The pretensioner integrates into the seat belt retractor and communicates with the SRS ECU via a dedicated yellow connector under the seat.

The B17801B fault code on the Seal U indicates an open circuit in the right middle-row seat belt pretensioner circuit, which may stop the pretensioner working correctly in a crash and can affect SRS protection. Severity is medium. Common causes include a loose or oxidised yellow SRS connector under the seat, or a broken wiring harness near the seat rail. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from 5 recorded cases.
5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Pretensioner wiring harness connector loose or contacts oxidized: Vehicle vibration, wading, or previous repairs loosen the yellow plug under the seat (usually marked 'SRS' or 'AIRBAG'), or terminal oxidation causes excessive resistance.
  • 2Wiring harness fatigue fracture at the seat slide rail: On models with a sliding right middle-row seat, the pretensioner wiring harness bends repeatedly during fore-and-aft seat adjustment, breaking the internal copper strands while the outer insulation appears intact (hidden open circuit).
  • 3Pretensioner internal igniter open circuit: Open circuit in the pretensioner internal pyrotechnic igniter or motor-driven pretensioning mechanism coil. Resistance measures infinite. Common causes include pretensioner aging, moisture ingress, or failure to replace the unit after deployment.
  • 4Improper repair after a vehicle accident: A collision caused the pretensioner to deploy and tighten, but technicians replaced only the seat belt assembly without replacing the pretensioner, or failed to clear the fault code after replacement.
  • 5Seat modification or non-standard parts: Failing to reconnect the pretensioner after removing it to install a non-OEM seat or leather upholstery, or installing an aftermarket seat without a pretensioner.
What To Do
  • 1
    Safety preparation: Turn off the ignition, disconnect the low-voltage battery (12V) negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor. For new energy vehicles, first disconnect the high-voltage service disconnect and verify the high-voltage system is de-energized.
  • 2
    Locate the component: Find the right second-row seat belt pretensioner, typically at the lower outboard side of the seat or below the B-pillar (varies slightly by model). Identify the dedicated yellow connector (with shorting bar protection).
  • 3
    Visual inspection: Inspect the connector for full engagement, the shorting bar for binding, and the wiring harness at the seat slide rail for wear or crush marks. Check for foreign objects under the seat obstructing the wiring harness.
  • 4
    Resistance measurement: Use a digital multimeter (requires low-current test function, <10mA) to measure the resistance between the pretensioner connector terminals. The normal value is 2.0-3.0 Ω (refer to the specific vehicle repair manual). If the resistance is infinite or >10 Ω, an open circuit exists in the pretensioner body or wiring.
  • 5
    Harness continuity test: Disconnect the SRS ECU connector (disconnect the battery first). Measure harness continuity between the pretensioner connector and the SRS ECU. Check for an open circuit or short to ground.
  • 6
    Repair action: If the connector is loose, reconnect and secure it; if the wiring harness is broken, repair or replace the harness (use high-temperature shielded wire); if the pretensioner is faulty, replace the seat belt assembly (the pretensioner is usually unavailable separately).
  • 7
    System reset: Reconnect all connectors and the 12V battery negative terminal. Use the VDS2000 or a BYD dedicated diagnostic tool to clear the fault code. Perform an 'SRS system self-check' or 'configuration check'. Verify DTC B17801B does not return and the SRS warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Wear on the right second-row seat rail severed the pretensioner wiring harness on the E3 ride-hailing vehicle.

✓
A BYD E3 rideshare vehicle with 120,000 km came in with the SRS warning light constantly illuminated. Scanned and retrieved DTC B17801B. Inspection revealed frequent seat sliding adjustments during commercial operation had chafed the wiring harness sheath at the right middle seat runner. Two internal conductors had completely fractured, though externally only minor scratches were visible. Repair: Cut out the damaged section, resoldered using 0.5 mm² high-temperature-resistant wire, insulated with double-layer heat-shrink tubing, and adjusted the harness routing to increase slack and prevent future chafing. Fault resolved.
BYD DTC AI Analysis

Qin EV pretensioner connector not plugged in after accident repair

✓
2019 BYD Qin EV. After a right rear collision, a non-authorised garage replaced the right rear door and seat. The owner noticed the SRS warning light on. Diagnosis revealed the right rear seat pre-tensioner connector was completely unplugged (technician likely forgot to reconnect it). Reconnected the yellow connector, confirmed the locking tab engaged, cleared the fault codes, and the system returned to normal. After accident repairs, check all SRS component connections.
BYD DTC AI Analysis

E2: Aftermarket seat replacement resulted in missing pretensioners

✓
The BYD E2 owner replaced the right-hand second-row seat assembly with an aftermarket unit after the original seat was damaged. After the replacement, the instrument cluster displayed an SRS warning. Inspection revealed the aftermarket seat lacked a seatbelt pretensioner and the connector hung loose. Solution: Installed an OEM seat assembly with the pretensioner, reconnected the wiring and configured the seat presence sensor parameters. DTC B17801B cleared.
BYD DTC AI Analysis

Pretensioner internal igniter open circuit due to aging

✓
An E3 parked for several days threw a B17801B immediately after startup. Connectors checked good and harness continuity was normal, but the pretensioner body measured infinite resistance. Disassembly found oxidized, fractured igniter terminals inside (likely from long-term humidity). Replaced the seatbelt pretensioner assembly (requires replacing the retractor as well). Fault cleared.
BYD DTC AI Analysis

SRS control unit software false positive (intermittent fault)

✓
Qin EV owner reported the SRS warning light comes on intermittently. Historical DTC B17801B present; current status shows historical fault. Checked wiring and resistance—normal. Suspected outdated SRS ECU software causing overly sensitive resistance threshold detection. Solution: Upgraded SRS control unit software to V2.1 or higher and cleared adaptive learning values. No recurrence after one month of monitoring.
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.