AU
B2A2C12

DTC B2A2C12 indicates a short to battery positive (B+) in the drive circuit of the passenger-side temperature control flap actuator (air mix motor/blend door actuator) — Seal U

Thermal Management System

DTC B2A2C12 indicates a short to battery positive (B+) in the drive circuit of the passenger-side temperature control flap actuator (air mix motor/blend door actuator).

In the BYD thermal management system, the air mix motor controls the hot and cold air mixing ratio via a PWM signal to adjust the passenger air outlet temperature.

A short to power means the resistance between the motor power supply or control line and the battery positive terminal drops abnormally (typically <1Ω).

This causes the HVAC ECU to detect an abnormal voltage (near 12V battery voltage instead of the normal 0-5V PWM signal or feedback voltage), triggering the overvoltage protection mechanism and stopping the motor drive.

This fault prevents passenger-side temperature adjustment, typically leaving the flap stuck in a fixed position or defaulting to maximum hot or cold.

Extreme cases risk wiring overheating, blown fuses, control module damage, or localized overheating.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Damaged internal coil insulation in the heating/cooling motor causes an inter-turn short circuit or short to power (common with motor aging or overheating from prolonged binding).
  • 2Wiring harness wear or pinching causes the power wire to short to the motor drive or signal wire (commonly occurs at the instrument panel frame edge, HVAC housing seam, or harness retaining clips).
  • 3Internal power driver chip (MOSFET or H-bridge) breakdown in the air conditioning controller (HVAC ECU) causes the output terminal to short to power.
  • 4Connector water ingress, corrosion, or deformed pins causing a short circuit between terminals (common causes: water ingress at the front passenger floor, A/C evaporator condensate leak, or improper car washing)
  • 5Incorrect wiring harness routing during repair, shorting the constant power wire (B+) to the motor control wire (PWM/feedback), or swapping the motor connector with the adjacent power connector.
What To Do
  • 1
    Fault Confirmation and Data Stream Reading: Use the VDS2000/3000 diagnostic tool to read the fault code and confirm B2A2C12 is currently present. Access the air conditioning system data stream to verify the Actual Position and Target Position of the Passenger Side Temperature Control Motor match. Check if the motor position feedback voltage is abnormally high (close to battery voltage).
  • 2
    Power off and test circuit insulation: Disconnect the 12V battery negative terminal. Unplug the air conditioning controller (HVAC ECU) and passenger air mix actuator connectors. Use a multimeter to measure the resistance between the actuator harness-side drive pin and power (B+). If the resistance is <1Ω, a short circuit exists. Simultaneously measure the resistance to ground to rule out a short to ground.
  • 3
    Component isolation diagnosis: Remove the front passenger temperature blend door motor (usually located on the right side of the HVAC unit). Measure the resistance between the motor terminals (normal range is typically 20-200Ω; refer to the workshop manual for exact specifications). Measure the insulation resistance between any terminal and the motor housing (must be >1MΩ). If the motor has an internal short circuit or insulation failure, replace the temperature blend door actuator.
  • 4
    Harness continuity and insulation check: Follow the harness routing beneath the instrument panel, focusing on wear-prone points where the harness passes through the firewall, instrument panel bracket, and HVAC housing. Inspect for harness damage, exposed copper strands, or contact points with the metal vehicle body. Use an oscilloscope or multimeter to measure harness continuity from the controller to the motor and insulation to the power supply. Repair any damaged harness and install anti-abrasion sleeving.
  • 5
    Controller output verification: If the wiring harness and motor are normal, reconnect the battery. Turn the ignition ON and measure the output voltage at the corresponding air conditioning controller pin. If the pin outputs 12V without an active PWM command, the internal driver circuit has shorted. Replace the HVAC control module (programming and matching required).
  • 6
    Reset and calibration: After replacing the faulty component, clear the fault code and use the diagnostic tool to perform the air conditioning flap position learning/calibration procedure (Air Mix Door Initialization). Verify the passenger side adjustment function and response speed at each setting from the lowest temperature (LO) to the highest temperature (HI).
Real-World Cases
BYD DTC AI Analysis

2018 BYD Song MAX: Passenger-side AC temperature uncontrolled, constantly blows hot air

✓
Symptom: Owner reports the passenger side air vent constantly blows hot air regardless of temperature knob position, while the driver side temperature adjustment works normally. VDS shows DTC B2A2C12. Live data indicates the passenger side blend door actuator position feedback voltage is stuck at 12V (normal: 0-5V varying with position). Removed and inspected the passenger side blend door actuator. Measured internal motor coil resistance at 0.3Ω (normal ~80Ω), with continuity between pins and housing. The internal coil burned out, causing a short to power. Diagnosis reveals the motor gear mechanism had been sticking, causing excessive drive current and overheating damage. Fix: Replaced the passenger side blend door actuator (see BYD parts catalogue for part number), cleared foreign objects from the HVAC ducting, and performed blend door position calibration. Temperature control function restored.
Original source ↗
BYD DTC AI Analysis

2017 Qin EV300 Intermittent A/C Fault with Wiring Harness Wear

✓
Symptom: Passenger-side AC temperature control intermittently failed on rough roads; normal on flat surfaces. History showed intermittent DTC B2A2C12. Diagnosis: Static measurements revealed no abnormalities. Shaking the dashboard wiring harness reproduced the fault. Inspection of the right-side dashboard frame harness found the blend door actuator power supply wire insulation chafed through on a sharp frame edge. Copper wires intermittently contacted the body (power coupling through other circuits), causing an intermittent short to power. Repair: Repaired the damaged wiring, rewrapped with insulation tape, rerouted the harness to avoid sharp edges, and fitted anti-chafe rubber sleeves. Cleared fault codes and test drove 50 km; fault did not return.
Original source ↗
BYD DTC AI Analysis

2019 BYD Qin Pro DM – Air conditioning suddenly failed while driving with burning smell

✓
Symptoms: While driving at high speed, the passenger side suddenly blew hot air with reduced airflow, accompanied by a slight burning smell. Stopped to inspect: the AC system had stopped working. Towed to the dealership; DTC B2A2C12 present (current). Diagnosis: Disconnected the motor connector and measured 0Ω resistance between the harness-side motor drive terminal and power supply, confirming a short. Unplugged the AC controller connector; the short disappeared, indicating an internal controller fault. Disassembled the HVAC controller and found the drive chip (H-bridge circuit) had failed, causing the output to short directly to 12V. Resolution: Replaced the AC control module assembly, wrote the controller coding and performed AC system initialization calibration. Fault resolved.
Original source ↗
BYD DTC AI Analysis

2019 Song MAX: Passenger-side temperature fails to adjust after fording

✓
Symptoms: After driving through water, passenger-side climate control temperature would not adjust and stayed at maximum cooling. Dashboard displayed air conditioning system fault. DTC: B2A2C12. Diagnosis: Inspected passenger-side floor harness. Found firewall waterproof grommet dislodged; water entered the blend motor connector and shorted the motor drive terminal to constant power (visible water stains and verdigris inside connector). Motor tested normal; harness shorted to power. Repair: Cleaned and dried harness connector thoroughly, cleaned pins with electrical contact cleaner, applied conductive anti-corrosion paste, replaced waterproof sealing ring, and reinstalled waterproof grommet. Recommended checking battery pack seal integrity (unrelated to this fault, but required after water ingress). Result: Fault code cleared; function restored.
Original source ↗
BYD DTC AI Analysis

2019 Qin Pro EV: AC short circuit fault after repair

✓
Symptoms: Dashboard replaced at a non-authorised workshop; passenger-side AC then failed. Scan tool showed DTC B2A2C12. Diagnosis: The temperature blend motor connector had been swapped with the adjacent blower motor power connector during repair. Both connectors share similar shells but different pinouts. This misconnection fed 12V directly into the motor signal feedback circuit, causing a short-to-power that nearly burned out the AC controller. Fix: Corrected the connector positions. Checked the controller and motor for damage (caught early; no hardware damage). Cleared the fault codes and performed the air door learning procedure. Fault resolved. Advised owner to use an authorised service centre for repairs.
Original source ↗
Other Seal U 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.