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 8

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.

The B2A2912 fault code on the Atto 8 indicates a high-severity short to 12V power in the HVAC defrost flap actuator drive circuit. This can disable windscreen defrosting and affect visibility. Common causes include a damaged defrost actuator motor or chafed under-dash wiring contacting a power feed. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Internal coil insulation damage 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 under-dash wiring harness 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 power supply wire).
  • 3Condensation leakage or water ingress from wading causes a short circuit between pins 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).
  • 4Improper wiring harness routing during a previous dashboard removal and installation caused a screw to pinch the harness, or a power wire mistakenly pierced the A/C wiring harness during the installation of aftermarket equipment such as a dashcam.
  • 5Electrical breakdown of the internal motor driver 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 the BYD dedicated diagnostic tool (VDS2000 or Launch X431) to read the complete DTCs and freeze frame data. Verify parameters such as ambient temperature and motor duty cycle at the time of the fault. Attempt to clear the fault code and observe if it recurs immediately.
  • 2
    Check the air conditioning system fuses in the engine compartment and under-dash fuse boxes (usually F1/10, F2/13, or 15A/20A fuses marked HVAC). If blown, replace with a fuse of the same rating. Do not 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
    Set a multimeter to resistance and measure the actuator-side connector. Normal defrost motor coil resistance is 50-150 Ω (stepper motor two-phase windings are approximately 100 Ω). If the measurement is near 0 Ω or less than 10 Ω, the actuator has an internal short circuit. Replace the actuator assembly (part numbers usually start with BC or EA).
  • 5
    If the actuator is normal, keep the actuator connector disconnected and measure continuity from the control wire at the harness-side connector to body ground and the constant power supply (B+). Under normal conditions, the control wire has no continuity to ground or the power supply (infinite resistance). If the control wire has continuity to the power supply, inspect the wiring harness along the circuit for damage.
  • 6
    Inspect the wiring harness protective sleeve for damage at the instrument panel crossmember fixing bolts, along the evaporator housing edge, and near ground points G101 or G102 connecting to the front compartment harness. Repair any damage and apply double protection using insulating tape and corrugated conduit.
  • 7
    Reconnect all connectors, clear the fault codes, and perform the air conditioning system self-learning procedure (via the diagnostic tool: Special Functions → Damper Self-learning). Test if the defrost mode switches smoothly and observe if the fault code returns.
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 Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 overview →
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.