AU
B1651-00

This DTC indicates the airbag control unit (SRS ECU) detects the front passenger seat belt pretensioner circuit resistance exceeds the calibrated upper limit (typically >3 — Seal U

Safety System

This DTC indicates the airbag control unit (SRS ECU) detects the front passenger seat belt pretensioner circuit resistance exceeds the calibrated upper limit (typically >3.5Ω, standard value approximately 2.0Ω±0.4Ω).

Electrically, the pretensioner is essentially a squib consisting of a resistance wire and an igniter charge.

High resistance usually indicates high impedance or an open circuit.

Potential causes include a partially blown internal resistance wire in the pretensioner, increased connector contact resistance, or a hidden open circuit in the wiring harness.

This fault causes the SRS system to enter degraded mode: during a frontal collision, the front passenger seat belt fails to pretension automatically, and the system may suppress the front passenger airbag deployment strategy, severely compromising passive safety performance.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal aging of the seat belt pretensioner assembly: Long-term vibration or thermal cycling oxidizes and thins the pretensioner igniter resistance wire, causing the resistance value to drift beyond the threshold.
  • 2Poor wiring harness connector contact: The yellow SRS connector under the seat or at the B-pillar (usually beside the front passenger seat rail or inside the lower B-pillar trim panel) has oxidized or loose terminals, or the shorting bar fails to fully disengage, increasing contact resistance.
  • 3Floor wiring harness damaged: Vehicle wading, carpet cleaning, or liquid spills in the front passenger area cause harness corrosion; frequent fore-and-aft seat adjustment causes harness fatigue and breakage (especially repeated bending from detached harness retaining clips under the seat).
  • 4SRS ECU sampling circuit error: A fault in the control unit internal A/D conversion circuit or sampling resistor causes resistance reading deviations (rule out through cross-checking).
  • 5Improper modification or repair: Replacing the front passenger seat with non-genuine parts, or splicing wires to retrofit a seat heating/ventilation system, introduces additional series resistance into the pretensioner circuit or causes poor connections due to damaged insulation.
What To Do
  • 1
    Safety preparation: Switch the vehicle to OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor. Wear an anti-static wrist strap and disable wireless communication devices.
  • 2
    Fault confirmation: Connect the VDS2000 or BYD dedicated diagnostic tool and read the fault code status. Confirm whether B1651-00 is 'Active' or 'History'. If multiple SRS fault codes exist, troubleshoot common power supply and ground issues first.
  • 3
    Physical location: Remove the front passenger seat (4 retaining bolts, torque usually 40-50 N·m) or the lower B-pillar trim panel and locate the yellow-marked pretensioner wiring harness connector (usually labeled F-P/T or PRETENSIONER FR RH).
  • 4
    Offline measurement: Disconnect the pretensioner connector. Use a digital multimeter (low-resistance range, 0.1 Ω accuracy) to directly measure the resistance across the two terminals on the pretensioner side. Normal value: 1.6–2.4 Ω. If the resistance is >3.0 Ω or infinite, the pretensioner has failed. Replace the seat belt assembly (do not replace the pretensioner separately).
  • 5
    Wiring harness inspection: If the pretensioner unit resistance is normal, check the connector pins for oxidation or blackening, and the female terminals for spreading. Measure harness continuity from the connector to the SRS ECU (located under the center console or front compartment). Resistance must be <1.0 Ω. Inspect the under-seat wiring harness for wear within the seat rail travel range.
  • 6
    Insulation check: Use a megohmmeter to measure the insulation resistance between the pretensioner circuit and body ground. The resistance must be >10MΩ. If the insulation value is low, inspect the wiring harness for chafing and short circuits to the body.
  • 7
    System reset: After repairing or replacing the faulty component, reconnect all connectors (listen for the locking 'click') and reinstall the seat. Connect the battery, switch the ignition to ON, use the diagnostic tool to clear the fault code, and perform 'SRS system configuration' (if installing a new component, write the VIN and configuration code).
  • 8
    Function verification: Perform an SRS system self-diagnosis and confirm B1651-00 does not recur. Verify the instrument panel airbag warning lamp turns off after the 6-second self-check. Use an SRS deployment simulation tool (such as a 10Ω resistor) to verify the ECU sampling circuit operates normally before conducting a vehicle test.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV450 front passenger pretensioner high resistance fault following water ingress

✓
A 2018 Qin EV450 had water on the passenger floor from a blocked sunroof drain. After the customer dried the carpet, the airbag warning light stayed on. The retrieved DTC was B1651-00. Inspection found water inside the SRS wiring harness connector under the passenger seat, and the pins had green copper corrosion. After cleaning the connector and applying conductive grease, the pretensioner resistance returned to 2.1 ohms, but the continuity resistance from the harness side to the ECU fluctuated between 0.8 and 2.5 ohms. The copper wires inside the floor wiring harness had corroded and broken at the water accumulation point. Replacing the floor wiring harness resolved the fault. Recommend clearing the sunroof drains to prevent recurrence.
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat modification on Qin 100 caused loose connection in pretensioner circuit

✓
2017 BYD Qin 100. Owner had the front passenger leather electric seat replaced at an outside shop with an aftermarket unit. One week later, DTC B1651-00 set. Inspection found the shop used non-standard crimp terminals when transferring the original seatbelt pretensioner connector, and didn't waterproof them. The pretensioner circuit had about 2.5Ω of contact resistance in series. The shop re-soldered the wiring harness, sealed it with heat shrink tubing, and installed a genuine connector. Resistance returned to normal. Reminder: Seat modifications involving SRS must use genuine wiring harness and terminals. Simple twisting or splicing is strictly prohibited.
BYD DTC AI AnalysisFrom Chinese market (translated)

Intermittent B1651-00 due to seat rail interference

✓
A Qin EV300. The customer reported the airbag warning light would occasionally illuminate, especially after adjusting the front passenger seat forward or backward. DTC B1651-00 was intermittent. Inspection found the wiring harness retaining clip beneath the seat had detached. The seat rail track repeatedly pinched the harness, causing some internal copper strands to fracture while the insulation remained intact, creating high resistance. Repaired the harness, re-secured the clip, and adjusted the wiring slack to provide enough free play at both full-forward and full-rearward seat positions. The fault did not recur. This case shows the importance of securing wiring harnesses.
BYD DTC AI AnalysisFrom Chinese market (translated)

Resistance drift due to pretensioner body aging

✓
2017 BYD Qin 80, 120,000 km, no collision history. DTC B1651-00 set suddenly. Measured resistance at the pretensioner connector: 4.8Ω, too high. Disconnected the connector and measured the pretensioner body directly (requires removing the seatbelt retractor), reading 4.9Ω. Determined to be internal resistor wire aging in the pretensioner. Replaced the passenger side seatbelt assembly (with pretensioner), clearing the fault. Analysis: Long-term high temperatures (vehicle parked outdoors long-term) caused oxidation of the igniter's internal resistor wire. For this type of aging fault, replace the complete assembly directly; do not attempt repair.
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. Sources: [1]