AU
B169D

DTC B169D indicates the SRS (Supplemental Restraint System) ECU (Electronic Control Unit) detected a critical fault during its internal self-check — Seal U

Safety System

DTC B169D indicates the SRS (Supplemental Restraint System) ECU (Electronic Control Unit) detected a critical fault during its internal self-check.

Specifically, this code indicates the airbag control module's internal monitoring circuit detected a processor fault, memory checksum error, power management circuit abnormality, or safety watchdog timeout.

As the core of the passive safety system, the SRS ECU monitors crash sensor data in real time, diagnoses system integrity, and precisely controls the deployment timing of the airbags, seat belt pretensioners, and active head restraints during a collision.

When B169D sets, the ECU cannot guarantee its decision-making logic reliability.

It enters fail-safe mode and disables all airbag deployment functions.

As a result, the airbags may fail to deploy during a frontal, side, or rear-end collision, and the instrument cluster airbag warning light remains illuminated.

Although the vehicle remains drivable, the significantly reduced passive safety protection creates a serious safety risk.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1SRS ECU internal integrated circuit fault: Physical damage, cold solder joints, or aging in the control unit's internal microprocessor, memory chip, or power supply regulator circuit causes the self-test to fail.
  • 2Abnormal power supply: battery voltage too low (<10V) or too high (>16V), poor contact in the SRS ECU constant power (B+) circuit, momentary interruption of the ignition power supply (IGN), or excessive resistance in the ground circuit (>1Ω), causing unstable ECU power supply or reset.
  • 3Communication bus fault: Diagnostic CAN bus (CAN-H or CAN-L) short to power or ground, open circuit, or abnormal terminating resistance (deviating from 60Ω), interrupting communication between the ECU and the vehicle network or causing data verification failure.
  • 4Collision sensor circuit fault: An intermittent short or open circuit in the front, side, or center impact sensor wiring harness triggers the ECU safety monitoring circuit alarm, resulting in a false control unit fault.
  • 5Software/calibration data corrupted: Electromagnetic interference, improper power-off, or write errors corrupt the control program or crash threshold calibration data in the ECU internal flash memory, causing abnormal logic operation.
What To Do
  • 1
    Initial diagnosis: Use VDS2000 or a BYD dedicated diagnostic tool to read the complete DTC list. Check for accompanying U-class (communication) or B16XX (sensor) fault codes, and record the voltage and mileage from the freeze frame data.
  • 2
    Power supply system check: Measure battery static voltage (standard 12.6V±0.2V) and voltage with the engine running (13.8-14.4V). Check voltage drop at the SRS ECU connector terminals (constant power B+, IGN power, GND). Voltage drop must be <0.1V and ground resistance must be <1Ω.
  • 3
    Communication line inspection: Disconnect the battery negative terminal and measure the CAN bus terminating resistance at the OBD diagnostic port (60Ω±2Ω). Power on the vehicle, then measure the static voltage and waveform of CAN-H (2.5-2.7V) and CAN-L (2.3-2.5V) to rule out signal interference.
  • 4
    Wiring harness and connector inspection: Locate the SRS ECU (usually under the center console, in front of the gear selector, or behind the glovebox). Inspect the connector for corrosion, looseness, or water ingress. Measure the continuity and insulation of the crash sensor wiring harness. Confirm there is no short to ground or short to power.
  • 5
    Software update and calibration: Update the SRS ECU to the latest software version (if a TSB is available). Perform the 'Write Vehicle Information' and 'Airbag System Calibration' functions to reconfigure the crash sensor parameters.
  • 6
    ECU replacement and programming: If the above steps fail, confirm an internal ECU fault. Disconnect the battery negative terminal, wait 3 minutes, and remove the old ECU. Install a new ECU with the same part number. Use the diagnostic tool to execute the 'Module Replacement' procedure, write the VIN, and perform configuration coding.
  • 7
    System verification: Reconnect all components, clear the fault codes, and perform a key cycle self-test. Connect an airbag simulator load (2Ω resistor) in place of the actual airbag, perform the ignition circuit test, and verify DTC B169D does not return and the warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Battery aging caused SRS ECU low-voltage reset in Tang DM.

✓
A 2021 Tang DM with 80,000 km displayed 'Check SRS System' on the instrument cluster. The scan tool retrieved DTC B169D. Inspection revealed battery voltage dropped to 9.2 V during cranking, causing the SRS ECU to reset repeatedly. After replacing the battery and cleaning the ground connections, the code cleared but returned intermittently. Further inspection found loose crimp terminals on the ECU power supply wiring. Re-crimping the terminals and applying conductive grease permanently resolved the fault.
BYD DTC AI Analysis

Song MAX: CAN bus interference causes false ECU fault

✓
A 2019 BYD Song MAX developed DTC B169D and an airbag warning light that stayed on after an aftermarket audio system installation. Diagnosis revealed the installer damaged the SRS wiring harness sheath when routing cables through the A-pillar, causing the CAN-H line to run parallel with the audio power cable and generate electromagnetic interference. The CAN bus waveform showed significant spikes. Reinsulating the harness and separating the communication lines from the high-voltage/high-current wiring cleared the fault.
BYD DTC AI Analysis

Yuan EV crash sensor wiring harness short circuit triggered ECU protection

✓
A 2019 Yuan EV logged DTC B169D after wading through water, accompanied by DTC B1650 (front impact sensor fault). Removing the front bumper revealed water ingress and corrosion on the left front impact sensor connector, with the wiring harness shorted to ground. The SRS ECU detected the sensor circuit abnormality, triggered its internal protection mechanism, and logged B169D. Replaced the left front impact sensor wiring harness assembly, dried the ECU connector (no water ingress), and recalibrated the sensor. The system then returned to normal.
BYD DTC AI Analysis

Qin Pro SRS ECU internal memory fault

✓
2020 Qin petrol variant. After accident repairs, the vehicle continuously reported DTC B169D that could not be cleared. Inspection found it had a previous front impact; the airbags were replaced but the SRS ECU was not. The SRS ECU contained locked crash records and a damaged memory block, so standard clearing could not reset it. Replaced the SRS ECU with a new module, performed online programming and crash sensor calibration, then ran the 'Crash History Clear' routine using the diagnostic tool. Fault resolved.
BYD DTC AI Analysis

Intermittent B169D caused by software version defect

✓
The airbag warning light on a 2021 Tang illuminated intermittently on rough roads. The scan tool retrieved history fault code B169D. Wiring inspection found no abnormalities; power supply was stable. The BYD TSB confirmed a software bug in the SRS ECU for this batch of vehicles that falsely reports internal faults at specific vibration frequencies. Flashed the ECU software to the March 2022 version. After a 100km road test, the fault did not recur.
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.