AU
B1691

DTC B1691 indicates the Airbag Control Module (ACM) detects a verification failure of its internally stored factory-encrypted data or security authentication key — Seal U

Safety System

DTC B1691 indicates the Airbag Control Module (ACM) detects a verification failure of its internally stored factory-encrypted data or security authentication key.

This fault involves the SRS security authentication mechanism.

The module triggers this fault when its stored factory configuration data, encryption key, or anti-theft authentication information does not match the reference data stored in the VCU/BCM.

Common causes include: installing a non-genuine airbag module without online matching, corrupted internal EEPROM data, vehicle power system anomalies causing encrypted data loss, or failing to complete the module matching procedure correctly during production or repair.

This safety-related fault causes the airbag system to enter a degraded mode and may limit normal airbag deployment.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Replacing the Airbag Control Module (ACM) with a non-genuine part or failing to match the ACM online causes an encryption key mismatch with the vehicle.
  • 2Electromagnetic interference, static electricity, or a hardware fault corrupted the factory configuration data or encryption key stored in the airbag module.
  • 3Severe vehicle battery discharge, improper jump-starting, or power supply system voltage fluctuations resulting in module data loss.
  • 4Using non-dedicated diagnostic equipment to operate the SRS system during repairs, inadvertently triggering factory mode or overwriting encrypted data.
  • 5Software upgrade failure or module hardware fault (such as EEPROM damage) causes security authentication algorithm verification failure.
What To Do
  • 1
    Use the BYD VDS diagnostic tool to read the complete fault codes and freeze frame data. Confirm whether B1691 is a current fault and record the vehicle status when the fault occurred.
  • 2
    Check the airbag control module (ACM) power supply (constant B+, ignition IG1), ground, and CAN-H/CAN-L circuit connections. Measure the supply voltage to confirm it is within 12V ± 0.5V, and verify the ground resistance is less than 1Ω.
  • 3
    Perform the 'SRS system online matching' or 'factory encryption reset' procedure. Verify and rewrite the module encryption data via the official BYD server to match the module with the vehicle VIN and anti-theft system.
  • 4
    If online matching fails, verify the module part number, hardware version, and software version match the vehicle configuration. If necessary, replace with a new genuine ACM and repeat the matching procedure.
  • 5
    After matching, clear the fault code and perform the 'SRS system self-check'. Confirm the airbag warning light turns off normally. Perform a battery disconnect test and a road test to verify the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

Replaced SRS module with salvaged unit on accident-damaged vehicle, now showing B1691.

✓
After a front-end collision, a workshop installed a used airbag control module (ACM) from the same model into a 2021 BYD Tang. The airbag warning light remained on. VDS diagnosis retrieved DTC B1691 (Factory Encryption), which would not clear. Inspection revealed the salvaged module had not been unbound. The workshop then fitted a new genuine ACM, performed online matching and immobiliser authentication via VDS, cleared the DTC, and restored normal airbag system function.
BYD DTC AI Analysis

Extended stationary period caused battery discharge, resulting in abnormal SRS data

✓
A 2019 BYD Yuan EV sat for about 3 months, causing severe battery discharge (voltage below 5V). After jump-starting, the instrument cluster displayed "Check SRS System". The scanner retrieved DTC B1691 and low-voltage history records. Charged the battery, replaced the aging battery, and used VDS to perform "SRS System Initialization" and "Clear History Faults". The DTC cleared. Low voltage caused a temporary module data anomaly.
BYD DTC AI Analysis

Software upgrade interrupted, causing encryption verification failure

✓
A 2020 BYD Qin SRS control module software update failed when the diagnostic tool lost connection mid-process. After reconnecting, the instrument cluster displayed the airbag warning light and stored DTC B1691. Reflashing the software had no effect. Using the VDS, the technician executed the 'Factory Encryption Reset' special function and then re-performed the module-to-vehicle identity authentication and data synchronisation, which cleared the fault.
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.