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B1621

DTC B1621 indicates the SRS (Supplemental Restraint System) electronic control unit detects an abnormally low-resistance path (typically <0 — Seal U

Safety System

DTC B1621 indicates the SRS (Supplemental Restraint System) electronic control unit detects an abnormally low-resistance path (typically <0.5Ω) between the driver side airbag squib circuit and body ground (GND).

Under normal conditions, airbag squib resistance must measure 2.0-3.0Ω, and insulation resistance to ground must exceed 1MΩ.

A short to ground indicates an unintended ground connection in the squib circuit.

This causes: 1) During a collision, the ground connection bypasses current, preventing proper airbag deployment and resulting in a loss of side-impact protection; 2) In extreme cases, wiring vibration may cause the ECU to misinterpret the short as a valid deployment signal, creating a risk of unintended airbag deployment; 3) The SRS system enters fail-safe mode, illuminates the instrument cluster airbag warning light, and disables the associated airbag function.

This fault affects a core active safety system component and requires immediate repair.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Driver seat side airbag wiring harness wear: Because the side airbag typically integrates into the seat backrest, the wiring harness routes near the seat slide rail to connect to the body harness. Long-term forward and backward seat adjustment causes the wiring harness to rub against the metal slide rail. Damaged insulation allows the copper core to short to body ground.
  • 2Displaced, corroded, or water-damaged internal terminals inside the yellow airbag connector under the seat (Clock Spring or Seat Connector) cause a short circuit between the terminals and the connector housing (ground). This commonly occurs after vehicle wading or deep interior cleaning.
  • 3Side airbag module internal squib short circuit: Moisture in the airbag igniter charge or a manufacturing defect causes the ignition wire to short to the metal housing.
  • 4SRS ECU internal detection circuit fault: Damaged internal A/D converter or monitoring chip in the control unit falsely reports a short-to-ground fault.
  • 5Improper modification or repair: Fixing screws pierce the airbag wiring harness when retrofitting seat ventilation/heating or installing non-OEM seat covers, or the seat slide rail crushes the wiring harness if not secured to standard after accident repairs.
What To Do
  • 1
    Safety preparation: Shift the vehicle into P, apply the parking brake, turn off the ignition switch, and disconnect the 12V battery negative terminal. Wait at least 90 seconds to fully discharge the SRS backup power supply and prevent accidental airbag deployment.
  • 2
    Fault Confirmation: Reconnect the battery. Use a dedicated BYD diagnostic tool (such as ED400 or VDS) to read the DTC. Confirm B1621 is a Current Code rather than a historical code. Record the Freeze Frame Data to check the vehicle speed and ambient temperature at the time of the fault.
  • 3
    Visual inspection: Remove the driver's seat (leave the wiring harness connected) and inspect the yellow airbag wiring harness under the seat (usually marked with yellow corrugated tubing or tape) for wear, damaged insulation, or crushing. Focus on areas where the harness contacts the seat slide rails and base frame.
  • 4
    Circuit isolation test: Turn off the ignition, disconnect the battery, and unplug the airbag connector under the seat (usually a yellow 2-pin connector). Use a multimeter set to resistance to measure the resistance from each of the two pins on the harness side (body to ECU direction) to body ground. Normal resistance is infinity (OL). A reading of <1Ω confirms a short to ground.
  • 5
    Step-by-step diagnosis: If the wiring harness side is normal, measure the resistance between the two pins on the airbag module side (seat side). Normal resistance is 2.0-3.0 Ω. A reading near 0 Ω indicates an internal short circuit in the airbag module. If the wiring harness side is shorted, inspect the wiring harness channel below the B-pillar along the harness routing for water marks, corrosion, or mechanical damage.
  • 6
    Connector inspection: Inspect the airbag connector pins for deformation, backed-out pins, or corrosion. Measure continuity between the pins and the connector housing to verify no short to ground. Inspect the SRS ECU connector (usually located under the centre console or centre tunnel) for water ingress or oxidation.
  • 7
    Repair verification: Repair the damaged wiring harness (solder and insulate with double-layer heat-shrink tubing) or replace the faulty airbag module/connector. Restore all connections. Clear the fault code. Perform a function test using a dedicated airbag simulator resistor (2.0Ω) instead of the actual airbag. Confirm the system passes the self-check and the fault code does not return.
  • 8
    Final confirmation: Remove the simulated resistor, connect the actual airbag, perform a system self-check, and road test the vehicle to verify the fault does not recur when adjusting the seat to its extreme positions.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Worn seat rails chafed the wiring harness on BYD Yuan EV, causing B1621.

✓
The airbag warning light on a 2019 BYD Yuan EV with 32,000 km was constantly on. A diagnostic tool read current fault code B1621. Inspection found 3 cm of wear on the yellow wiring harness beneath the driver's seat, at the frontmost position of the seat rail, with exposed copper contacting the rail and creating a short to ground. Cause: The owner frequently adjusted the seat, causing the wiring harness retaining clip to detach. The harness then rubbed against the rail. Action: Repaired the harness by soldering the wires and applying double-layer heat shrink tubing. Rerouted the harness, secured it with cable ties, and ensured more than 20 mm clearance between the harness and seat rail. Fault cleared.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Tang DM airbag connector shorted due to water ingress after wading.

✓
2021 BYD Tang DM. After wading, the instrument cluster displayed 'Check SRS System'. Scanned DTC B1621-00 (short to ground). Removed the driver seat and found obvious water stains inside the airbag connector underneath; pins had oxidised and turned black. Measured pin-to-ground resistance at 0.3Ω. Cause: Water entered through the sill seal during wading, pooled under the seat, and shorted the connector. Repair: Thoroughly cleaned the connector, removed corrosion with electronic cleaner, blew it dry, applied conductive grease, and replaced the waterproof connector seal. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro: internal short circuit in airbag module after seat replacement

✓
2020 BYD Qin petrol variant. DTC B1621 appeared after a leather seat installation at an auto parts market. The original seat airbag harness was forcibly pulled, damaging it internally. The old seat airbag measured 0.8Ω (internal short to ground). The new seat airbag measured 2.4Ω (normal), but during the transfer a screw pierced the harness, causing a short to ground. Repair: Replaced the airbag module with an OEM unit, repaired the damaged harness section, and routed the wiring properly to avoid contact with the seat frame. Cleared the fault and advised the customer to avoid unprofessional modifications.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song MAX SRS ECU false alarm

✓
2019 Song MAX with intermittent airbag warning light. Scan tool showed intermittent B1621 fault code. Checked wiring harness and airbag resistance – both normal. Fault difficult to reproduce. Detailed inspection revealed the SRS ECU mounting bracket was slightly deformed, causing the internal circuit board to contact the metal bracket when driving over bumps, creating a spurious ground path. Repair: Adjusted ECU mounting position, fitted insulating washers, tightened screws to standard torque (8 N·m). Monitored for one week with no recurrence. Confirmed false trigger caused by abnormal grounding between ECU housing and body.
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]