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) — Atto 3

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 Atto 3 indicates an open circuit in the right middle-row seat belt pretensioner circuit, so the SRS system may not tighten that belt correctly in a crash. Severity is medium because restraint performance can be reduced. Common causes include a loose or corroded yellow SRS connector under the seat, or damaged wiring near the seat rail. 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 — Atto 3
  • 1Loose pretensioner wiring harness connector or oxidized contacts: The yellow plug under the seat (usually marked 'SRS' or 'AIRBAG') loosened from vehicle vibration, water ingress, or previous repairs, or terminal oxidation caused excessive resistance.
  • 2Wiring harness fatigue fracture at the seat slide rail: On vehicles with a sliding right middle-row seat, fore-aft adjustment repeatedly bends the pretensioner wiring harness. This breaks the internal copper strands while the insulation remains visually intact (hidden open circuit).
  • 3Pretensioner internal igniter open circuit: The internal pyrotechnic igniter or motor-driven pretensioning mechanism coil has an open circuit. Measured resistance is infinite. Pretensioner aging, moisture ingress, or failure to replace a deployed pretensioner usually cause this.
  • 4Improper repair after a vehicle accident: A collision caused the pretensioner to deploy and retract, but the technician replaced only the seat belt assembly, not the pretensioner, or failed to clear the fault code after replacement.
  • 5Seat modification or non-standard replacement parts: Removing but failing to reconnect the pretensioner when installing a non-OEM seat or reupholstering the seat in leather, or installing an aftermarket seat without a pretensioner.
What To Do
  • 1
    Safety preparation: Turn off the ignition switch, disconnect the low-voltage battery (12V) negative terminal, and wait at least 90 seconds to ensure the SRS capacitor fully discharges. For new energy vehicles, first disconnect the high-voltage service disconnect and confirm the high-voltage system powers down.
  • 2
    Locate the component: Find the right middle-row seat belt pretensioner, usually located at the lower outboard side of the seat or below the B-pillar (varies slightly by model), and identify the dedicated yellow connector (with shorting bar protection).
  • 3
    Visual inspection: Verify the connector is fully seated. Check if the shorting bar is stuck. Inspect the wiring harness at the seat slide rail for wear or pinch marks. Check for foreign objects under the seat obstructing the wiring harness.
  • 4
    Resistance measurement: Use a digital multimeter (must have a 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 vehicle workshop manual). Infinite resistance or >10 Ω indicates an open circuit in the pretensioner body or wiring.
  • 5
    Harness continuity test: Disconnect the battery, then disconnect the SRS ECU connector. Measure harness continuity between the pretensioner connector and the SRS ECU, and 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 VDS2000 or a BYD dedicated diagnostic tool to clear the fault code. Perform the "SRS system self-check" or "configuration check". Confirm 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 Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 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.