AU
B16A4

DTC B16A4 indicates the airbag control unit (SRS ECU) detected an internal system fault or critical support circuit abnormality — Seal 6 EV

Safety System

DTC B16A4 indicates the airbag control unit (SRS ECU) detected an internal system fault or critical support circuit abnormality.

As the core of the passive safety system, the ECU processes real-time data from the front and rear crash sensors, side pressure sensors, and accelerometers.

It determines airbag deployment, seat belt pretensioner activation, and high-voltage interlock disconnection.

This DTC triggers when the ECU self-diagnostic detects an internal CPU processing error, an EEPROM memory checksum failure, a transient voltage drop at the power supply monitoring circuit, or a communication interruption between the master and slave safety chips.

In this condition, the system enters fail-safe mode.

This mode may disable all airbag deployment functions and fails to guarantee normal protection logic during a collision, creating a severe safety hazard.

The B16A4 fault code on the Seal 6 EV indicates an internal fault in the airbag control unit (SRS ECU), such as a processing error or power supply problem. Severity is medium. Common causes include cold solder joints on the internal circuit board or power supply circuit faults. Visit an authorised dealer for repair. Typical repair cost data is available based on 4 recorded cases.
4
Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Cold solder joint on the SRS ECU internal PCB, or hardware fault in the main control chip (e.g., Infineon or Renesas series)
  • 2ECU power supply circuit fault: excessive resistance in the constant power (+B) circuit, poor contact at the ignition switch power supply (IG1), or corrosion at ground terminals G301/G302 causing voltage fluctuation.
  • 3CAN bus communication fault: Electromagnetic interference affects the Power CAN or Chassis CAN signals between the SRS ECU and the instrument cluster or VCU, or mismatched terminating resistance causes data frame loss.
  • 4Crash sensor internal short circuit: Front or side crash sensor signal harness shorted to ground or power, triggering ECU analog input port overload protection.
  • 5Internal ECU accelerometer damage from a previous collision, or vehicle wading causing ECU housing seal failure, resulting in internal circuit board oxidation and corrosion.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (ED400 or VDS) to read the complete DTC list, check for accompanying B16A3 (SRS ECU internal fault), U0155 (lost communication with instrument cluster), or B1B00-series sensor fault codes, and record the freeze frame data.
  • 2
    Check the battery voltage and body ground points. Measure the voltage at SRS ECU connector terminal 1 (+B constant power) and terminal 9 (IG1 power). Verify the static voltage is ≥12.4V and the dynamic voltage is ≥9V. Verify the ground circuit resistance is <1Ω.
  • 3
    Disconnect the battery negative terminal and wait 3 minutes (for capacitor discharge). Check the ECU connector (located under the center console or at the rear of the center armrest box) for pin oxidation, backed-out terminals, or wiring harness wear. Clean with electrical contact cleaner if necessary.
  • 4
    Use an oscilloscope to measure CAN-H (2.5-3.5V) and CAN-L (1.5-2.5V) waveforms, check for abnormal pulses or shorts to power or ground, and verify network topology integrity.
  • 5
    Check the SRS ECU software version and compare it with the BYD TIS technical bulletin. If a newer software version exists (such as a patch for B16A4), update the software and reconfigure the vehicle VIN and airbag parameters.
  • 6
    If the fault code persists, replace the SRS ECU assembly (select the part number by vehicle model, e.g., Yuan EV: EG-5820100). After replacement, perform: ① Coding ② Crash sensor calibration ③ System self-diagnostic test.
  • 7
    Use the diagnostic tool to perform the 'Airbag System Check' function and simulate a crash signal (without actual ignition). Confirm system status returns to normal, clear the fault code, and road test to verify.
Real-World Cases
BYD DTC AI Analysis

BYD Tang DM-i: Power fluctuation causes intermittent SRS ECU fault

✓
A 2021 Tang DM-i with 30,000 km on the clock showed an intermittent "Check SRS System" warning on the dash, with fault codes B16A4 and U0155 present. Diagnosis revealed an aftermarket dash cam drawing power from the SRS fuse circuit, causing voltage to drop to 8.2V on startup. Restored the factory power circuit, fitted a filter capacitor to the SRS ECU power supply, and upgraded the ECU software to V2.3 to improve voltage tolerance. Fault resolved.
BYD DTC AI Analysis

Flooded Yuan EV: SRS ECU Internal Corrosion Repair Case

✓
2019 Yuan EV logged DTC B16A4 after a water wading incident. Disassembly found the SRS ECU mounted under the floor. The internal PCB showed copper corrosion, and the accelerometer chip solder joints had cracked. Because the ECU is a safety-critical component and internally potted, it cannot be repaired; we fitted a new ECU. Recalibrated the zero points for the three collision sensors (left front, right front, centre) using the diagnostic scanner, and completed the high-voltage interlock loop check. Total labour time: 4.5 hours.
BYD DTC AI Analysis

CAN bus interference caused B16A4 fault in BYD Song MAX

✓
After the accident repair, the SRS warning light stayed on and stored DTC B16A4 along with multiple U communication faults. Inspection found the wiring harness was not secured properly during the repair, causing the SRS ECU CAN bus wiring to run parallel to the orange high-voltage wiring for an excessive distance. This caused electromagnetic coupling interference. Fix: re-routed the wiring to maintain ≥200 mm separation between the CAN lines and high-voltage cables, added a ferrite core at the ECU end, cleared the fault codes, and monitored the vehicle for 200 km with no recurrence.
BYD DTC AI Analysis

Failure to configure the instrument cluster after replacement caused an SRS fault on the Qin Pro EV

✓
Following instrument cluster replacement at a dealership, a 2020 Qin Pro EV set an SRS B16A4 fault with the airbag warning light remaining constantly on. Diagnosis revealed the replacement cluster lacked the correct security system configuration code, causing a failed security authentication handshake with the SRS ECU. Repair procedure: Using the VDS diagnostic tool, navigate to 'Special Functions - SRS Configuration', enter the correct configuration code (based on vehicle trim: 5-airbag or 6-airbag variant), and perform online matching to clear the fault.
Other Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.