AU
B2A1212

This DTC indicates the Thermal Management Control Unit (TMS) or air conditioning control module detects a short circuit in the signal circuit of the high-pressure side pressure sensor for the plate heat exchanger (Chiller/battery cooler) refrigerant circuit — Seal U

Thermal Management System

This DTC indicates the Thermal Management Control Unit (TMS) or air conditioning control module detects a short circuit in the signal circuit of the high-pressure side pressure sensor for the plate heat exchanger (Chiller/battery cooler) refrigerant circuit.

The sensor outputs an abnormal voltage (near 0 V when shorted to ground, or near 5 V or 12 V when shorted to power), exceeding the normal 0.5–4.5 V signal range.

This piezoresistive sensor monitors real-time refrigerant circuit pressure (typically 0–3.5 MPa) to control the Electronic Expansion Valve (EXV) opening and regulate compressor speed.

The short circuit eliminates system pressure feedback and prevents accurate refrigerant flow control.

This fault can trigger thermal management derating: limiting battery fast-charging power, prohibiting high-power discharge, and reducing motor power output.

In extreme cases, the loss of overpressure protection can cause pipe rupture or compressor damage.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Worn wiring harness insulation causing a signal wire short to ground (commonly at the front compartment firewall pass-through, harness clip wear points around the battery pack, or inside the underbody shield).
  • 2Moisture-induced short circuit on the internal circuit board of the pressure sensor body (sensor seal failure allowing refrigerant or moisture to enter).
  • 3Short circuit between connector terminals (water ingress into the connector after wading causes terminal electrolytic corrosion, shorting the signal wire to the ground wire or power wire)
  • 4Improper repair operations damaged the wiring harness (during previous chassis or front compartment repairs, a sharp metal edge cut the wiring harness and remained unrepaired).
  • 5Short circuit in thermal management control unit internal signal processing circuit (ECU internal A/D conversion circuit or signal conditioning circuit fault; relatively rare)
What To Do
  • 1
    Connect VDS or a dedicated diagnostic tool, read the fault code status and freeze frame data, confirm the pressure sensor voltage value (a short circuit is usually 0V or 5V reference voltage), and record the ambient temperature and system status at the time of the fault.
  • 2
    Disconnect the refrigerant pressure sensor connector near the plate heat exchanger (usually on the right side of the front compartment or at the front of the chassis battery pack) and measure the resistance on the sensor side. Normal resistance: 2 kΩ–10 kΩ (depending on specific model). If resistance is <100 Ω, replace the sensor assembly.
  • 3
    If the sensor is normal, measure the wiring harness side: Turn the ignition ON and measure the voltage between the signal wire and ground. If the voltage is 0V, check for a short to ground; if it is 5V/12V, check for a short to power. Disconnect the TMS/A/C controller connector and measure the insulation resistance of the signal wire to ground and to power. The resistance must be >10MΩ.
  • 4
    Perform a wiring harness continuity test: measure continuity between the sensor connector signal terminal and the corresponding TMS pin (normal: <1 Ω). Visually inspect the wiring harness, focusing on wear points in the front compartment high-temperature zone, inside the chassis undertray, and at the battery pack mounting brackets.
  • 5
    Check connector sealing: Inspect terminals for back-out, burning, or green corrosion, and measure insulation resistance between terminals. If water ingress is evident, clean and dry the connector. Replace the sealing ring or wiring harness assembly if necessary.
  • 6
    Substitution test: Connect a known good sensor of the same type. Observe if the data stream returns to normal (normal pressure varies with ambient temperature, typically 0.8-1.5MPa@25℃) to confirm the fault location.
  • 7
    After completing the repair, clear the fault code, calibrate the refrigerant pressure (if required by the workshop manual), and start the thermal management system self-test. Verify the deviation between the actual pressure and the sensor feedback value is within ±0.05MPa. Road test the vehicle to verify normal battery cooling/heating functions.
Real-World Cases
BYD DTC AI Analysis

Qin EV plate heat exchanger pressure sensor shorted after water ingress

✓
After driving through water, the dashboard reported a thermal management system fault. Retrieved DTC B2A1212. Found water inside the refrigerant pressure sensor connector at the front of the underfloor battery pack. Electrolytic corrosion between terminals had shorted the signal wire to ground. Disassembled the connector and found green copper corrosion. Wiring harness continuity tested normal. Cleaned the terminals thoroughly, dried with compressed air, applied electronic contact protectant, and replaced the seal ring. Fault cleared. Check underbody shield sealing to avoid driving through water again.
BYD DTC AI Analysis

Wiring harness wear caused intermittent short circuit

✓
2019 BYD Qin EV intermittent thermal management fault while driving, DTC B2A1212 intermittent. Found wiring harness sheath damaged where it routes from the front bulkhead to the chassis. On rough roads, the signal wire intermittently shorted to chassis ground. Insulation resistance normal at rest but dropped sharply when shaking the harness. Repaired the damaged harness, re-wrapped with abrasion-resistant corrugated tubing, and adjusted retaining clip positions to avoid sharp edges. Fault completely resolved.
BYD DTC AI Analysis

Internal short circuit in the sensor

✓
AC not cooling; thermal management warning light on. Scanner read B2A1212. Disconnected the sensor connector: harness side voltage normal (4.8V on signal wire), but resistance across the sensor pins measured only 15Ω—shorted. Removed the sensor and found the internal circuit board shorted from long-term corrosion due to refrigerant micro-leakage. Replaced the refrigerant pressure sensor, recovered and recharged the refrigerant, ran a system pressure test (hold test), and delivered after confirming no leaks.
BYD DTC AI Analysis

Improper wiring harness routing after repair caused short circuit

✓
The vehicle presented with fault code B2A1212 following a battery pack replacement at a non-authorised repairer. Inspection revealed the technician had pinched the refrigerant pressure sensor wiring harness between the battery pack mounting bracket and the body. Prolonged compression damaged the insulation and caused a short to ground. Re-routed the harness to avoid the pinch point and secured it with genuine retaining clips. Repaired the damaged conductors using solder and heat-shrink tubing. Restored standard routing and cleared the fault.
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.