AU
B1694

DTC B1694 indicates the airbag control unit (SRS ECU) detects its operating supply voltage is below the system threshold (usually 9 — Seal U

Safety System

DTC B1694 indicates the airbag control unit (SRS ECU) detects its operating supply voltage is below the system threshold (usually 9.0V-9.5V, depending on vehicle calibration).

The SRS system uses a dual power supply design: constant power (+B, memory power) and ignition switch power (IG).

This ensures airbag deployment during a collision even if the ignition switch is off.

Low voltage prevents normal operation of the airbags, seat belt pretensioners, crash sensors, and CAN communication.

The system enters fail-safe mode (the instrument cluster airbag warning light remains illuminated).

In extreme cases, this causes deployment failure during a collision or risks unintended deployment.

New energy vehicles use a DC-DC converter instead of a traditional alternator for low-voltage charging.

Abnormal DC-DC output or high-voltage system power loss also triggers this code.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Low-voltage battery aging or deep discharge: Battery state of health (SOH) below 70% and insufficient cold cranking amps (CCA) cause static voltage to drop below 12.0V or an excessive voltage drop under dynamic load.
  • 2Poor contact in the SRS power supply circuit: Increased contact resistance caused by an oxidized SRS fuse (typically F1/14 or F2/03) in the instrument panel junction box (EJB), a loose fuse socket, or water ingress and oxidation at the SRS ECU connector (32-pin/48-pin).
  • 3DC-DC converter fault (new energy vehicles): DC-DC output voltage falls below 13.5V or remains unstable, failing to maintain the 12V power supply for the low-voltage system. High-voltage interlock faults or DC-DC module damage commonly cause this on models such as the Tang DM-i and Song PLUS EV.
  • 4SRS control unit internal power management IC fault: Damage to the ECU internal voltage regulation circuit, buck converter chip, or filter capacitor causes a false low-voltage reading (actual circuit voltage is normal).
  • 5Excessive parasitic current draw: Aftermarket devices such as dash cams or GPS trackers cause quiescent current to exceed 30 mA, dropping battery voltage below 10 V after long-term parking.
What To Do
  • 1
    Read freeze frame: Use the VDS 2000/3000 diagnostic tool to access the SRS system and record the freeze frame data for DTC B1694. Review the 'Battery Voltage' value and mileage at the time of the fault to confirm if it is an intermittent fault.
  • 2
    Battery inspection: Use a battery tester to measure CCA and SOH. Static voltage must be ≥12.4V and cranking voltage must be ≥9.6V. If voltage is below 12.0V or SOH <70%, charge or replace the battery (requires ≥60Ah AGM specification). Clear the fault code and road test the vehicle.
  • 3
    Fuse and power distribution box inspection: Check if the SRS fuse (15A/20A) in the under-instrument panel fuse box (EJB) is blown and if the socket is burnt. Measure the voltage drop across the fuse; it must be <0.1V. Check the constant power supply circuit in the engine compartment fuse box (EE/B).
  • 4
    Harness and connector measurement: Disconnect the negative battery terminal, wait 90 seconds, and unplug the SRS ECU connector (usually located under the center console or behind the armrest box). Measure the voltage between connector pin 1 (+B constant power) and body ground; it must be 12V. Measure the voltage at connector pin 2 (IG power) with the ignition ON; it must be 12V. Verify the tightening torque of ground point G104 (under the left A-pillar) or G202 (under the right A-pillar) is 9N·m. Sand the ground point if necessary.
  • 5
    DC-DC output verification (new energy vehicle): Power the vehicle to READY mode. Use a multimeter to measure the voltage across the battery positive and negative terminals. The voltage must be between 13.8V and 14.2V. If the voltage is below 13.0V, check the DC-DC converter high-voltage input and low-voltage output circuits, and check the BMS for accompanying high-voltage interlock fault codes.
  • 6
    Load test: Connect the diagnostic tool to read the live data stream. Operate high-power electrical consumers such as headlights, air conditioning, and seat heating. Monitor the SRS system 'Power Supply Voltage' for a sudden drop (a drop exceeding 2V indicates a poor circuit connection).
  • 7
    Control unit replacement: If all above checks are normal but the fault code recurs, the SRS ECU has an internal fault. Replace the airbag control unit (part number varies by vehicle model, e.g., SC-3636100). Perform online matching (write the VIN and configuration code) and calibrate the crash sensor.
Real-World Cases
BYD DTC AI Analysis

2019 BYD Yuan EV: Degraded auxiliary battery caused SRS system to enter sleep mode

✓
Vehicle parked for 3 days. On startup, the instrument cluster displayed "Check Airbag System". Retrieved DTC B1694 (active fault). Static battery voltage measured only 11.2V, CCA at just 40% of rated value. Diagnosis found the owner frequently drives short trips and had installed a dashcam with 24-hour monitoring (static current 45mA). Resolution: Replaced 60Ah AGM battery (LN2-3703010), disconnected dashcam from constant power and wired to ACC, cleared DTC using VDS, and performed SRS self-test. Fault resolved.
BYD DTC AI Analysis

2021 Tang DM-i: Abnormal DC-DC converter output causing intermittent fault codes

✓
The airbag warning light illuminated intermittently while driving. B1694 was a stored fault code, alternating with B1695 (supply voltage too high). Freeze frame data showed voltage fluctuating between 8.5V and 15.8V. Checked the low-voltage battery – normal. Measured the DC-DC converter output terminal; voltage dropped below 12.0V under increased load. Further inspection found the DC-DC low-voltage output wiring harness connector (located on the front left longitudinal beam) had internal pin retraction, with contact resistance of 2.3Ω. Repaired the connector pins, applied conductive paste, and secured the wiring harness to prevent vibration. The fault has not recurred.
BYD DTC AI Analysis

2020 BYD Qin Pro petrol: water ingress and oxidation at the SRS ECU connector

✓
After driving in rain, the airbag warning light stayed on constantly. The scan tool showed DTCs B1694 and B1650 (airbag warning lamp circuit fault). Inspected the vehicle—no accident history. Battery voltage 12.6V, normal. Removed the centre console and found the SRS ECU under the right side of the air conditioning evaporator. The 32-pin connector had obvious water stains and green corrosion. The air conditioning drain hose had detached, causing condensate to leak inside. Cleaned the SRS ECU connector pins using electrical contact cleaner, replaced the air conditioning drain hose, wrapped the connector with waterproof tape and rerouted the wiring harness to avoid the water source. Cleared the fault codes and the system returned to normal.
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.