AU
B1788

DTC B1788 indicates a short to power (B+) in the Stage 2 inflator circuit of the driver-side dual-stage front airbag — Seal U

Safety System

DTC B1788 indicates a short to power (B+) in the Stage 2 inflator circuit of the driver-side dual-stage front airbag.

The SRS control unit continuously monitors the airbag deployment circuit resistance (normally 2.0–3.0 Ω) using internal safety sensors and diagnostic circuits.

The control unit identifies a short to power when the Stage 2 airbag circuit voltage continuously exceeds the threshold (typically over 80% of supply voltage) or the resistance drops abnormally low (near 0 Ω).

This fault causes: 1) Stage 2 airbag deployment failure during a collision, reducing occupant protection; 2) Accidental deployment risk, where the airbag triggers without a collision; 3) The SRS system to enter fail-safe mode, inhibiting all airbag functions.

This fault involves a high-voltage deployment circuit and represents a highest-level safety fault.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Broken internal conductor or damaged insulation in the clock spring (spiral cable) shorts the second-stage airbag wiring harness to the power supply line. This commonly occurs in vehicles over 5 years old or subject to frequent steering wheel rotation.
  • 2Driver airbag module (DAB) internal second-stage igniter (Squib) insulation failure; prolonged vibration, temperature cycling, or manufacturing defects cause an internal short circuit.
  • 3The SRS wiring harness wears against sharp edges on the steering column or instrument panel frame. The damaged insulation contacts vehicle power supply lines (such as the ignition switch power supply or instrument backlight power supply).
  • 4Water ingress, oxidation, or bent pins in the airbag connector (usually located under the steering wheel or on the airbag module) cause a short circuit between adjacent pins, especially after driving through water or in high-humidity environments.
  • 5Fault in the SRS control unit (ACU) internal diagnostic or drive circuit causing a false short circuit report, or internal power transistor breakdown causing a short circuit between the output terminal and the power supply.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds to discharge the residual charge in the SRS capacitor. Attach an 'Airbag Under Repair' warning label to the steering wheel to prevent accidental operation.
  • 2
    Fault confirmation: Connect the diagnostic tool to read and record all SRS fault codes. If B1788 is present, check the Freeze Frame data to confirm parameters such as vehicle speed and ambient temperature when the fault occurred. Attempt to clear the fault code. If the fault code remains Current and fails to clear, confirm a hardware fault.
  • 3
    Initial visual inspection: Inspect the airbag wiring harness connector under the steering wheel (usually yellow) for looseness, water ingress, or obvious burn marks. Inspect the clock spring connector pins for bends or corrosion.
  • 4
    Circuit isolation test: Remove the driver airbag module (use the special tool and follow the 'point away from body' principle). Connect an airbag simulator (Dummy Load/2.7Ω resistor) to the wiring harness in place of the airbag. Reconnect the battery and turn the ignition on. If the fault code changes to a history code or clears, the airbag module is faulty. If the fault code remains, the wiring harness or clock spring is faulty.
  • 5
    Circuit continuity and insulation test: Disconnect the SRS control unit connector. Use a multimeter to measure continuity between the airbag second-stage wiring harness and the power supply (B+). The normal condition is open circuit (OL). If continuity exists, inspect the wiring harness section by section. Focus on wear-prone areas such as inside the steering column sleeve and the instrument panel bracket mounting points.
  • 6
    Clock spring test: Disconnect the clock spring-to-airbag connector. Measure the second-stage circuit resistance across both ends of the clock spring (steering wheel side and body harness side). Resistance must be less than 1 Ω. Measure the insulation resistance between this circuit and the power terminal. Resistance must be greater than 10 MΩ. If insulation is poor, replace the clock spring.
  • 7
    SRS control unit diagnosis: If the wiring harness, clock spring, and airbag module are normal but the fault persists, check the SRS control unit power supply and ground. If the power supply is normal, replace and reconfigure the SRS control unit.
  • 8
    Repair verification: After replacing the faulty component, clear all fault codes and perform the SRS system self-diagnosis. Perform a static test (ignition switch ON, check if the airbag warning light turns off) and a dynamic test (simulate a crash signal, use the diagnostic tool to execute the 'Crash Output Test' function, and confirm the Stage 2 circuit resistance is within the normal range).
Real-World Cases
BYD DTC AI Analysis

Internal short in clock spring caused DTC B1788 on BYD Tang DM

✓
Model: 2021 Tang DM, Mileage: 45,000 km. Symptom: Airbag warning light constantly illuminated. Scan tool retrieved active DTC B1788. Repair: Disconnected battery. Found slight water ingress at the clock spring connector below the steering wheel. Removed the clock spring; multimeter measured insulation resistance between secondary airbag circuit (Pins 3-4) and power circuit (Pin 5) at only 0.3Ω, confirming internal layer-to-layer short. Cause: Long-term parking in humid underground garage degraded insulation, allowing internal copper foil to short against power leads. Replaced clock spring (Part number: BYD-5823010). Fault resolved.
BYD DTC AI Analysis

Yuan EV airbag module internal igniter short circuit

✓
2019 Yuan EV535, 82,000 km. Airbag warning light illuminated suddenly while driving; no collision history. Intermittent DTCs B1788 and B1787 (short to ground). Repair: Airbag simulator substitution. Fault cleared with simulator connected, confirming harness integrity. Disassembled driver airbag module and found trace metal powder between second-stage igniter pins. Resistance measured normal after cleaning, but fault recurred after reinstallation. Replaced driver airbag module (first- and second-stage igniters). Fault eliminated. Root cause: Insulation breakdown between bridge wire and housing inside airbag igniter due to aging.
BYD DTC AI Analysis

Song MAX: Aftermarket steering wheel modification damaged wiring harness

✓
Vehicle: 2019 Song MAX, 35,000 km. Modifications: Aftermarket sports steering wheel installed. Symptoms: Airbag warning light on. Customer had previously removed and reinstalled the steering wheel at a modification shop. Repair: Found the driver's airbag stage 2 harness (yellow wire, normally paired with black) pinched by a retaining clip under the steering column. Insulation ruptured, exposing copper wire that contacted the dashboard frame (ground) and a nearby red power wire (steering wheel heating modification harness), shorting to power. Repaired the harness (rewrapped with insulation tape, rerouted), removed the illegal heating harness. Fault cleared. Note: Steering wheel modifications easily damage the clock spring and airbag harness.
BYD DTC AI Analysis

Qin Pro SRS control unit false positive

✓
Vehicle: 2020 Qin Pro (petrol), 12,000 km. Symptom: After accident repairs (front bumper and left front fender replaced), the airbag warning light stayed on constantly with DTC B1788. Repair: Measured airbag module resistance at 2.4Ω (normal). Checked wiring harness insulation to power supply (normal). Monitored second-stage loop voltage with an oscilloscope and detected a 12V spike lasting approximately 5 ms at ignition ON. Inspection revealed the SRS control unit connector (located at the front of the center tunnel) was not fully seated. Reseated and secured the connector; the voltage spike disappeared. Cleared the DTC; fault did not return. Determined cause: poor contact at the control unit connector caused the diagnostic circuit to falsely trigger.
BYD DTC AI Analysis

Tang EV connector corrosion after driving through water

✓
Model: 2021 Tang EV, Mileage: 28,000 km. Symptom: Airbag warning light illuminated the day after the vehicle drove through standing water approximately 30 cm deep. Repair: Read DTC B1788 (current). Removed and inspected the driver-side airbag connector; found obvious water stains and green corrosion inside the yellow connector. After cleaning the connector, measured a persistent short-to-power on the second-stage circuit. Further inspection revealed water had penetrated the clock spring, causing an internal short. Replaced the clock spring and all airbag connectors exposed to water (including the floor harness side), and dried the SRS system completely. Apply airbag-system-specific waterproofing agent to the connector seals.
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.