AU
B17121A

DTC B17121A indicates the Supplemental Restraint System (SRS) detects a 0-ohm resistance in the right curtain airbag ignition circuit — Atto 3

Safety System

DTC B17121A indicates the Supplemental Restraint System (SRS) detects a 0-ohm resistance in the right curtain airbag ignition circuit.

Under normal operating conditions, the curtain airbag squib requires a specific resistance (typically 2.0–3.0 ohms, depending on the vehicle model).

A 0-ohm resistance indicates a short circuit.

Possible causes include an internal short in the curtain airbag module, a short between the positive and negative harness wires, or a short between connector terminals.

This fault causes the SRS control unit to determine the curtain airbag has deployed or has a circuit fault.

Consequently, the right curtain airbag fails to deploy during a collision.

This severely compromises side impact protection and constitutes a high-risk safety fault.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Right curtain airbag igniter internal short circuit: A short circuit in the igniter bridge wire or pyrotechnic charge inside the curtain airbag module drops resistance to 0 ohms.
  • 2Damaged harness insulation causing short circuit: Right roof wiring harness (A-pillar to C-pillar section) worn through during vehicle use, modification, or accidents, causing direct contact between positive and negative conductors.
  • 3Connector water ingress and corrosion: Vehicle wading or a blocked sunroof drain hose causes water to enter the right curtain airbag connector (usually located on the roof side rail or above the A-pillar), resulting in a short circuit between terminals.
  • 4Modification damage: Mounting screws or tools pierce the wiring harness insulation and cause a short circuit during dashcam installation, headliner modification, or A-pillar trim panel replacement.
  • 5SRS control unit (ACU) internal detection circuit fault: Abnormal internal sampling circuit causes a false short circuit report (relatively uncommon).
What To Do
  • 1
    Safety preparation: Turn the vehicle OFF, disconnect the low-voltage battery negative terminal, and wait at least 3-5 minutes (some models require more than 5 minutes) to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Connect the diagnostic tool to read the freeze frame data. Confirm B17121A is a current fault (Present), not a history fault (History), and record the vehicle status when the fault occurred.
  • 3
    Visual inspection: Remove the right A-pillar trim, B-pillar trim, and right roof trim. Inspect the right curtain airbag module connector for looseness, water ingress, corrosion, or obvious burn marks. Inspect the wiring harness for damage or crush marks.
  • 4
    Curtain airbag resistance measurement: Disconnect the right curtain airbag connector. Use a digital multimeter (low-resistance range) to measure the resistance directly between the two pins on the curtain airbag side (gas generator side). The normal value is 2.0-3.0 Ω. If the reading is 0-0.5 Ω, the curtain airbag has an internal short circuit. Replace the right curtain airbag assembly.
  • 5
    Wiring harness insulation check: Keep the curtain airbag disconnected. Measure the resistance between the two pins on the wiring harness side (leading to the ACU). Resistance must exceed 10 MΩ (open circuit). Continuity indicates a short circuit in the wiring harness. Separately measure the insulation resistance of each wire to ground (body ground) and to power (12V) to confirm no short to ground or power.
  • 6
    Wiring harness continuity check: Use a multimeter in continuity mode to measure the circuit from the right curtain airbag connector to the SRS control unit connector. Verify normal continuity (resistance <1Ω). Focus inspection on the A-pillar hinge area and the roof wiring harness flex section.
  • 7
    Fault repair: If the curtain airbag is faulty, replace the genuine right curtain airbag assembly. If the wiring harness is faulty, repair or replace the damaged section. Use high-temperature flame-retardant wiring (airbag-specific wire) and wrap it correctly.
  • 8
    System verification: Restore all connections, connect the battery, use the diagnostic tool to clear the fault codes, and perform the 'SRS System Self-check' or 'Configuration and Settings' procedure. Confirm B17121A does not reappear and the airbag fault light turns off. Perform a simulated crash test (if equipment is available) to verify system function.
Real-World Cases
BYD DTC AI Analysis

Case: Yuan Plus right side curtain airbag inflator replacement due to internal short circuit

✓
A 2022 BYD Yuan Plus (Atto 3) had the SRS warning light staying on constantly on the instrument panel. The technician retrieved DTC B17121A with a VDS2000 diagnostic tool. Following standard procedure, the technician disconnected the battery, removed the right-side headliner trim, and disconnected the right curtain airbag connector. Measuring the curtain airbag side with a multimeter showed 0.2Ω (normal value approximately 2.3Ω), indicating an internal short in the inflator. The technician replaced the right-side curtain airbag assembly (Part No.: SC2E-5824710), cleared the fault code, and performed a system self-test, resolving the fault. Analysis suggests that some curtain airbags from this production batch may have internal bridgewire welding defects causing micro-short circuits.
Original source ↗
BYD DTC AI Analysis

Qin EV connector corrosion and short circuit repair case after water wading

✓
A 2019 Qin EV's SRS warning light came on the day after driving through water during heavy rain. DTC B17121A stored as a current fault. Inspection found a blocked sunroof drain tube, allowing water to flow from the A-pillar into the right side curtain airbag connector (above the right front A-pillar). Disassembly revealed the connector held water and the copper terminals showed green corrosion with short-circuiting. After cleaning the terminals with anhydrous ethanol, drying the harness with an air gun, and applying conductive grease before reconnection, the fault temporarily disappeared. It returned one week later. Replacing the right side curtain airbag wiring harness connector and unblocking the drain tube permanently fixed the issue.
BYD DTC AI Analysis

E3 wiring harness damaged by aftermarket dash cam installation

✓
After installing a dash cam (power drawn from the headliner), a 2020 BYD E3's airbag warning light illuminated. Diagnosis revealed DTC B17121A. Inspection found that the installer had pierced the right-side curtain airbag harness sheath with a self-tapping screw while securing the dash cam wiring, shorting the positive and negative wires. After removing the screw, we found damaged insulation on both wires with copper cores touching. We cut out the damaged section, repaired it by soldering with special high-temperature flame-retardant TXL 18AWG wire, applied double-layer heat shrink tubing, and rerouted the wiring to avoid the curtain airbag area. Fault cleared.
BYD DTC AI Analysis

Right side curtain airbag wiring harness aging, chafing, and short circuit case

✓
A 2019 Qin EV with 120,000 km on the clock and no accident history: the SRS warning light suddenly came on. Scanning revealed DTC B17121A. Inspection showed the right curtain airbag connector was normal, and the airbag resistance measured 2.1 ohms. Tracing the wiring harness revealed that at the roof rail above the B-pillar, a cable clip had detached, allowing the harness to chafe against the body metal edge until the insulation wore through and shorted. Repair: replaced about a 30 cm section of the harness, re-routed and secured it to keep clear of sharp edges, and fitted an anti-chafing sleeve. Fault resolved.
BYD DTC AI Analysis

Case: ACU replacement due to false SRS control unit fault

✓
A 2020 Model E3 repeatedly logged DTC B17121A. The right curtain airbag resistance measured 2.2Ω (normal), and the wiring harness insulation was intact with no shorts. After disconnecting the airbag, we measured abnormally low resistance between the ACU output pins. We traced the fault to an internal right curtain airbag drive circuit issue in the SRS control unit (ACU), causing a false short circuit. We replaced the airbag control unit (VIN matching and system configuration required), then performed 'coding' and a 'collision output test' (simulation test) using the dedicated diagnostic scanner. This permanently cleared the fault.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.