AU
B2A2912

This fault code indicates abnormal continuity between the air conditioning system defrost flap actuator (Mode Actuator/Defrost Motor) drive circuit and the vehicle positive power supply (12V/B+) — Atto 3

Thermal Management System

This fault code indicates abnormal continuity between the air conditioning system defrost flap actuator (Mode Actuator/Defrost Motor) drive circuit and the vehicle positive power supply (12V/B+).

In BYD new energy vehicles, the defrost motor is a stepper motor or DC servo motor that drives the mode flap inside the HVAC housing to switch between face, foot, defrost, and other air distribution modes.

When the air conditioning control module (ACU) or body control module (BCM) detects a short to power in this motor control circuit, it triggers DTC B2A2912 and enters protection mode.

The module stops drive output to the motor to prevent control chip burnout or wiring harness fires.

This fault disables the front windshield defrost function, severely compromising driving visibility and safety in low-temperature or high-humidity environments.

Secondary risks include blown air conditioning system fuses or control module overheating.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Damaged internal coil insulation in the defrost flap actuator causes a short circuit between the positive and negative terminals, or a seized motor bearing causes an abnormal current rise that the system misdiagnoses as a short circuit.
  • 2The wiring harness under the dashboard chafes against a sharp metal body edge, damaging the insulation and causing the motor drive wire to contact a constant power wire (such as the dashboard fuse box supply wire).
  • 3Condensation leakage or vehicle wading causes a short circuit between terminals in the connector near the air conditioning evaporator housing (usually a 4-pin or 6-pin white plug), especially between PIN1 (power supply) and PIN3 (motor drive).
  • 4During a previous repair, improper wiring harness routing during dashboard removal and installation caused a screw to pinch the harness, or a power wire mistakenly pierced the air conditioning wiring harness during the installation of a dashcam or other aftermarket equipment.
  • 5Electrical breakdown of the internal motor drive chip (usually an H-bridge driver IC) in the air conditioning control module (ACU) causes the output terminal to continuously output a high level.
What To Do
  • 1
    Use a BYD dedicated diagnostic tool (VDS2000 or Launch X431) to read the complete DTC and freeze frame data. Verify parameters at the time of the fault, such as ambient temperature and motor duty cycle. Attempt to clear the fault code and observe if it immediately reappears.
  • 2
    Check the air conditioning system fuses in the engine compartment and under-dash fuse boxes (usually F1/10, F2/13, or the 15A/20A fuse marked HVAC). Replace any blown fuse with one of the same rating. Never install a higher-rated fuse.
  • 3
    Remove the glove box or center console to access the HVAC assembly. Locate the defrost mode door actuator (usually on the right side of the evaporator housing, with a white plastic linkage) and disconnect its electrical connector.
  • 4
    Measure the actuator-side connector using a multimeter set to resistance. The normal defrost motor coil resistance is 50-150 Ω (stepper motor two-phase windings are approximately 100 Ω). A measurement close to 0 Ω or less than 10 Ω indicates an internal short circuit in the actuator. Replace the actuator assembly (part numbers usually start with BC or EA).
  • 5
    If the actuator is normal, leave the actuator connector disconnected and measure continuity from the harness-side connector control wire to body ground and constant power (B+). Under normal conditions, the control wire has no continuity to ground or power (infinite resistance). If continuity to power exists, inspect the wiring harness along the circuit for damage.
  • 6
    Inspect the wiring harness protective sleeve for damage at the dashboard crossmember retaining bolts, the edge of the evaporator housing, and near ground points G101 or G102 connecting to the front compartment harness. Repair any damaged areas and double-protect them using insulating tape and corrugated conduit.
  • 7
    Reconnect all connectors, clear the fault code, and perform the air conditioning system self-learning procedure (using the diagnostic tool, navigate to Special Functions → Air Flap Self-learning). Test the defrost mode switch for smooth operation and check if the fault code returns.
Real-World Cases
BYD DTC AI Analysis

2018 BYD Song MAX: Defrost mode inoperative with blown fuse

✓
The customer reported that after activating front defrost mode, the AC panel displayed abnormally and the defrost function then failed completely. The technician used VDS to read DTC B2A2912 and found the instrument panel fuse box F1/10 (10A) blown. The replacement fuse blew again instantly. The technician disconnected the passenger side footwell trim panel and found the defrost damper actuator (part number BC-8112300) connector wiring harness cut by the sharp edge of the dashboard mounting bracket. The purple (motor drive +) and red (constant 12V) wires had damaged insulation that contacted each other, creating a short. The technician repaired the wiring insulation, added a rubber protective sleeve, and replaced the actuator (internally damaged by overcurrent). Fault resolved.
BYD DTC AI Analysis

2019 Qin EV: Unable to switch air conditioning modes after water ingress

✓
After driving through heavy rain and standing water, the instrument cluster displayed "Air Conditioning System Fault" and the defrost button did not respond. The scan tool retrieved codes B2A2912 and B2A2914 (defrost motor short to ground) present. Inspection of the lower HVAC assembly connector found obvious water traces with green oxidation on the pins. Disassembly showed the defrost actuator had an enclosed waterproof design and was undamaged, but the connector seal had deteriorated, allowing water entry and causing a short between the power and signal pins. Cleaned the connector with electrical contact cleaner, dried the wiring harness, replaced the waterproof seal and actuator connector. Fault resolved. Check the scuttle panel drain holes for blockages.
BYD DTC AI Analysis

Qin EV300: Sudden climate control failure following aftermarket modification

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After installing a 360-degree surround-view camera at an external modification shop, the customer found the AC only blew to the face vents and would not switch to defrost mode the next day, with a slight burning smell. Scanning showed DTC B2A2912. Inspection revealed the installer had driven self-tapping screws into the dashboard to secure the wiring harness; one screw pierced the defrost motor harness (yellow-green wire), shorting it to constant power. The damaged section sat behind the evaporator housing in a tight space. Technicians cut out the damaged length, soldered in replacement wire of the same gauge, insulated the splice with heat-shrink tubing, and rerouted the harness away from the screw holes.
BYD DTC AI Analysis

2019 Song MAX HVAC control module internal fault

✓
No external damage or water wading history. Demist function failed intermittently with DTC B2A2912. Previous repairs replaced the demist actuator and wiring harness without success. Detailed inspection found the fault only occurred with AC cooling on and ambient temperature above 35°C. Actuator circuit measurements normal. Diagnosis: AC control module (ACU, passenger side dashboard, PN 5A-8111030) internal H-bridge driver chip has poor thermal stability; high temperatures caused breakdown and output short to power. Replaced the AC control module and performed online programming via BYD Di-Link authorised account. Fault completely resolved.
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.