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B1621

DTC B1621 indicates the SRS (Supplemental Restraint System) electronic control unit detects an abnormally low-resistance path (typically <0 — Atto 3

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 — Atto 3
  • 1Driver seat side airbag wiring harness wear: The side airbag typically integrates into the seat backrest, requiring the wiring harness to route near the seat slide rail to connect to the body harness. Repeated forward and backward seat adjustment rubs the wiring harness against the metal slide rail. Damaged insulation exposes the copper core, shorting it to body ground.
  • 2Internal terminal displacement, corrosion, or water ingress in the yellow airbag connector under the seat (Clock Spring or Seat Connector) causes a short circuit between the terminal 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: A 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 piercing the airbag wiring harness during installation of seat ventilation/heating or aftermarket seat covers, or the seat slide rail crushing the wiring harness due to failure to secure it 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 fault (Current Code) rather than a historical fault. 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 (keep 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 crush marks. Focus on the contact points between the wiring harness, seat slide rails, and base frame.
  • 4
    Wiring isolation test: Switch 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 (towards the ECU) to body ground. Normal resistance is infinity (OL). A reading <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 has a short circuit, inspect the wiring harness channel below the B-pillar along the routing for water stains, 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 central 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 conduct a road test 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 Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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]