AU
B1799-00

DTC B1799-00 indicates the SRS (Supplemental Restraint System) ECU detects the front passenger-side second-stage seat belt pretensioner circuit resistance exceeds the normal upper limit (typically >3 — Seal U

Safety System

DTC B1799-00 indicates the SRS (Supplemental Restraint System) ECU detects the front passenger-side second-stage seat belt pretensioner circuit resistance exceeds the normal upper limit (typically >3.0-3.5Ω, normal range is 2.0-3.0Ω).

The second-stage pretensioner is a critical component of the airbag system.

During a severe collision, a pyrotechnic charge generates a rapid tightening force, working with the first-stage pretensioner to provide enhanced occupant restraint.

Excessive resistance typically indicates a high-resistance circuit contact, a partial open circuit, or an aging internal resistance wire within the pretensioner.

This causes the ECU to identify a pretensioner failure risk, illuminate the airbag warning light, and potentially disable the front passenger-side airbag and pretensioner functions.

During a collision, the second-stage pretensioner may fail to deploy properly, allowing excessive forward occupant movement and increasing injury risk.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Seat belt pretensioner internal resistance element aging or near-open circuit: Long-term use or moisture ingress causes the pretensioner squib resistance wire value to drift and exceed the ECU monitoring threshold.
  • 2Mechanical damage to the under-seat wiring harness: adjusting the front passenger seat forward or backward causes the seat bottom to rub against or pinch the frame-mounted pretensioner wiring harness, resulting in partial breakage of the internal copper wires or poor contact.
  • 3Poor connector connection: The pretensioner connector below the B-pillar or under the seat exhibits pin oxidation, looseness, water ingress corrosion, or a failed locking tab, causing increased contact resistance.
  • 4Non-genuine part resistance mismatch: The resistance value of an aftermarket seat belt assembly pretensioner installed after accident repairs does not match the factory calibration, or the repair used a wiring harness repair part with a different resistance value.
  • 5Harness excessively bent or improperly secured: Vehicle modifications (such as installing floor coverings or seat covers) pinched the pretensioner harness, or incorrect harness routing during accident repairs caused increased local resistance.
What To Do
  • 1
    Connect the VDS2000 or VDS6000 diagnostic tool to the vehicle. Read the DTC B1799-00 status to confirm if it is a current or history fault. Record the resistance value from the freeze frame data (typically 3.5-5.0 Ω).
  • 2
    Disconnect the 12V battery negative terminal and wait at least 90 seconds to allow the SRS backup power supply to fully discharge, ensuring safe operation and preventing accidental airbag deployment.
  • 3
    Locate the front passenger second-stage seat belt pretensioner (on Qin models, usually inside the lower B-pillar trim or at the seat frame connection). Visually inspect the connector for looseness, the terminals for oxidation or discoloration, and the inside of the connector for water stains or corrosion.
  • 4
    Measure the resistance across the pretensioner terminals using a multimeter. The standard value is 2.0±0.1Ω to 3.0±0.1Ω (refer to the vehicle repair manual for specific values). If the measured value is >3.5Ω, proceed to the next diagnostic step.
  • 5
    Disconnect the pretensioner connector and directly measure the pretensioner resistance. If the resistance is normal (2.0-3.0Ω), check the wiring harness. Inspect the harness near the seat slide rail fixing bolts and under the carpet for crushing or damage. Use a multimeter to measure harness continuity and resistance (should be <1Ω).
  • 6
    If the wiring harness is damaged, replace the connector or repair the damaged wire using the genuine wiring harness repair kit. Verify solder joint resistance is <0.5Ω. Apply waterproof tape and corrugated conduit to reinforce protection. If the pretensioner unit resistance is too high, replace the front passenger seat belt assembly (including the second-stage pretensioner) with a genuine part. Do not use aftermarket parts.
  • 7
    Reconnect all connectors. Ensure the connector locking tabs fully engage (listen for a 'click'). Restore the battery connection.
  • 8
    Clear the fault code using the diagnostic tool. Perform the SRS system self-check procedure and confirm B1799-00 does not return. Read the data stream to verify the pretensioner resistance value is within the normal range.
  • 9
    Perform a road test. Simulate bumpy road conditions and seat adjustments. Confirm the airbag fault warning lamp no longer illuminates and the system returns to normal.
Real-World Cases
BYD DTC AI Analysis

Qin Pro DM pretensioner internal resistance aging caused high resistance.

✓
At 80,000 km, the airbag warning light stayed on constantly. VDS read DTC B1799-00. The technician measured front passenger second-stage pretensioner resistance at 3.5Ω, outside the standard 2.0-3.0Ω range. The B-pillar lower harness connector showed no oxidation or looseness, and seat wiring had no damage. Diagnosed the fault as pretensioner internal resistance wire aging causing resistance drift. Replaced the OEM front passenger seatbelt assembly. Post-repair resistance measured 2.2Ω. Cleared the fault codes; system self-check passed and the warning light went out.
Original source ↗
BYD DTC AI Analysis

Song MAX seat track pinches wiring harness, causing intermittent high resistance

✓
Airbag warning light illuminates intermittently, typically on cold starts and clearing after driving some distance. VDS logged B1799-00 as an intermittent fault. The technician performed a harness wiggle test; wiggling the wiring beneath the passenger seat reproduced the fault. Disassembly revealed the seat rail mounting bolt crushing the seatbelt pretensioner harness, partially fracturing the internal copper wires. Contact resistance fluctuated between 2.5-4.5Ω with seat position changes. Repaired the damaged harness, rerouted the wiring clear of the seat rail travel range, and protected it with waterproof tape and corrugated tubing. Fault resolved.
Original source ↗
BYD DTC AI Analysis

Resistance mismatch in aftermarket parts after Qin EV accident repair

✓
After front collision repairs, the airbag warning light stayed on. VDS read B1799-00 (resistance too high) and B1798-00 (resistance too low). The passenger seatbelt assembly was an aftermarket part; the pretensioner measured 3.8Ω and the connector was loose. Replaced it with a genuine OEM unit, seated the connector fully and locked it, and measured 2.1Ω. Cleared the codes and the SRS self-test passed. Use genuine OEM SRS parts for accident repairs.
BYD DTC AI Analysis

Loose connector lock tab on Tang DM caused contact resistance fluctuation

✓
Airbag warning light intermittently illuminated on rough roads; normal on flat surfaces. VDS read B1799-00 as a history fault. Inspection found the passenger-side second-stage pretensioner connector lock loose—vehicle vibration caused slight plug displacement and poor contact. Oscilloscope monitoring caught resistance spiking to 4.2Ω during the fault. Replaced the pretensioner connector using a genuine wiring harness repair kit, secured the connection, and applied conductive grease to the pins to prevent oxidation. Road testing on rough surfaces after repair confirmed the fault cleared.
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.