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+) — Qin Plus

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.

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Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 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 misinterprets as a short circuit.
  • 2The wiring harness under the instrument panel chafes against sharp metal body edges, damaging the insulation and causing the motor drive wire to contact a constant power wire (such as the instrument panel fuse box power supply wire).
  • 3Condensate leakage or vehicle wading causes a short circuit between terminals in the connector near the A/C 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 routing during instrument panel removal and installation caused a screw to pinch the wiring harness, or a power wire accidentally pierced the air conditioning wiring harness during the installation of a dash cam or other aftermarket equipment.
  • 5Breakdown of the motor driver chip (usually an H-bridge driver IC) inside the air conditioning control module (ACU) causes the output terminal to continuously output a high level.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS2000 or Launch X431) to read the complete DTCs and freeze frame data. Verify the ambient temperature, motor duty cycle, and other parameters when the fault occurred. Attempt to clear the fault code and observe if it reappears immediately.
  • 2
    Check the air conditioning system fuses in the fuse boxes in the engine compartment and under the instrument panel (usually F1/10, F2/13, or 15A/20A fuses marked HVAC). If blown, replace with a fuse of the same rating. Never install a higher-rated fuse.
  • 3
    Remove the glove box or center console to expose 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. Normally, the defrost motor coil resistance is between 50 and 150 Ω (stepper motor two-phase windings are approximately 100 Ω). A reading close to 0 Ω or less than 10 Ω indicates an internal short circuit in the actuator. Replace the actuator assembly (part numbers usually begin with BC or EA).
  • 5
    If the actuator is normal, keep the actuator connector disconnected and measure the continuity of the control wire at the harness-side connector to body ground and constant power (B+). Normally, the control wire shows no continuity to ground or power (infinite resistance). If the wire shows continuity to power, inspect the wiring harness along the circuit for damage.
  • 6
    Inspect the wiring harness protective sleeve for damage at the instrument panel crossmember retaining bolt, the evaporator housing edge, and near the G101 or G102 ground point connecting to the front compartment wiring harness. Repair any damaged areas and apply double protection using insulating tape and corrugated conduit.
  • 7
    Reconnect all connectors, clear the fault code, and perform the air conditioning system self-learning procedure (via the diagnostic tool: Special Functions → Air Flap Self-learning). Test if the defrost mode switches smoothly and observe if the fault code reappears.
Real-World Cases
BYD DTC AI Analysis

2018 BYD Song MAX: Defrost mode inoperative with blown fuse

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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

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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

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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 Qin Plus 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.