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+) — Seal U

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 Seal U indicates a short to 12V power in the air conditioning defrost flap actuator drive circuit. This is a high-severity fault because it can disable front windscreen defrosting and reduce visibility. Common causes include a failed defrost actuator motor or damaged wiring under the dashboard contacting a power supply wire. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 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 increase that the system misinterprets as a short circuit.
  • 2The wiring harness below the dashboard chafes against sharp metal body edges, damaging the insulation. This causes the motor drive wire to contact a constant power wire (such as the dashboard fuse box supply wire).
  • 3Short circuit between pins at the connector near the A/C evaporator housing (usually a 4-pin or 6-pin white plug) due to condensation leakage or vehicle wading, 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 mistakenly punctured the air conditioning wiring harness during the installation of aftermarket equipment such as a dashcam.
  • 5Internal breakdown of the motor drive chip (typically an H-bridge driver IC) inside the air conditioning control module (ACU) causes a continuous high-level output at the terminal.
What To Do
  • 1
    Use a BYD dedicated diagnostic tool (VDS2000 or Launch X431) to read all DTCs and freeze frame data. Verify the ambient temperature, motor duty cycle, and other parameters at the time of the fault. Attempt to clear the fault codes and observe if they immediately return.
  • 2
    Check the air conditioning system fuses in the engine compartment and under-dashboard fuse boxes (usually F1/10, F2/13, or 15A/20A fuses 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 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. Normal defrost motor coil resistance is between 50 and 150 Ω (stepper motor two-phase windings are approximately 100 Ω). If the reading is close to 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, leave the actuator connector disconnected and measure continuity from the harness-side connector control wire to body ground and the constant power supply (B+). Under normal conditions, the control wire shows no continuity to ground or the power supply (infinite resistance). If the wire shows 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 mounting bolts, the edge of the evaporator housing, and near the G101 or G102 ground points connecting to the front compartment wiring 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 → Air Flap Self-learning). Verify smooth switching to defrost mode 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

✓
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 Seal U Fault Codes
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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.