AU
B123B17

This DTC indicates that, in the thermal management system of a BYD new energy vehicle, the monitored voltage on the high-voltage side of the air conditioning compressor exceeds the system safety threshold (typically >420V DC; the specific threshold varies by vehicle calibration) — Seal U

Thermal Management System

This DTC indicates that, in the thermal management system of a BYD new energy vehicle, the monitored voltage on the high-voltage side of the air conditioning compressor exceeds the system safety threshold (typically >420V DC; the specific threshold varies by vehicle calibration).

In the electric scroll compressor system of BYD Qin series models, the "high-voltage side" specifically refers to the high-voltage DC bus voltage driving the compressor motor, not the refrigerant pressure.

The Battery Management System (BMS) via the high-voltage interlock (HVIL) circuit, or the Air Conditioning Controller (ACU), triggers this protective DTC upon detecting an abnormally high voltage at the compressor inverter input.

This fault forces the air conditioning compressor to shut down, resulting in a loss of cabin cooling.

If the vehicle uses a battery liquid cooling system, this fault severely impairs the traction battery’s heat dissipation capability and may force the battery thermal management system to operate in a degraded mode.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal voltage sampling circuit fault in the A/C compressor controller (inverter), or high-voltage filter capacitor aging and failure, causing voltage detection drift.
  • 2Insulation degradation or partial short circuit in the electric compressor three-phase winding, causing abnormal bus voltage reflection.
  • 3Internal cell voltage imbalance in the traction battery pack or a BMS voltage detection channel fault causes the high-voltage output to exceed the threshold.
  • 4Oxidation or looseness of the air conditioning system high-voltage wiring harness connector (HVIL circuit), or poor contact at the interlock pins, causing abnormal voltage drop detection.
  • 5DC-DC converter or on-board charger (OBC) fault causes high-voltage bus voltage fluctuations, indirectly affecting voltage stability on the air conditioning high-voltage side.
What To Do
  • 1
    Use the BYD VDS 1000+ diagnostic tool to read complete fault codes and freeze frame data. Record key data streams including 'A/C high-side voltage', 'battery pack total voltage', 'compressor speed', and 'HVIL status'. Confirm the operating condition during the overvoltage event (charging/driving/idling).
  • 2
    Check the front compartment electric compressor high-voltage wiring harness connector (usually marked AC HV+/-). Verify high-voltage interlock (HVIL) circuit continuity (normal resistance <10Ω). Inspect the connector for signs of burning, terminal back-out, or water ingress.
  • 3
    Use an insulation tester to measure the insulation resistance between the electric compressor high-voltage input terminal and the housing (standard: >500MΩ). Use a multimeter to compare the total BMS voltage with the actual voltage at the compressor terminal. The voltage difference must be <5V.
  • 4
    Check the A/C high-pressure sensor (located on the high-pressure side of the liquid line) signal voltage to rule out a false fault caused by a sensor short to power. Disconnect the sensor connector. Measure the signal wire voltage to ground (should be 0V) and to the power supply (should be 5V reference).
  • 5
    Perform an in-depth diagnosis of the traction battery pack. Check the cell voltage difference (<30mV) and insulation resistance to rule out overvoltage protection caused by an internal high-voltage fault in the battery pack.
  • 6
    If the wiring and sensor are normal, replace the electric compressor controller (inverter assembly) or update the ACU software to the latest version. Perform the 'Compressor Zero Point Calibration' and 'Air Conditioning System Self-Learning' procedures. Clear the fault code and road test to verify.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV300 compressor scroll seal failure triggered high-pressure protection

✓
The air conditioning intermittently stopped cooling. VDS stored DTC B123B17 accompanied by B123A11 (compressor overcurrent). Live data showed the high-side voltage spiking above 450V. Stripped down the electric compressor and found the scroll plate's internal seal had perished, causing leakage. This led to abnormal compressor load and increased motor back EMF, which tripped the high-voltage overvoltage protection. Replaced the upgraded scroll plate assembly (with added seal structure), added 120ml POE68 EV-specific compressor oil, reinstalled the compressor aligned to the original marks, and performed zero-point calibration. Fault cleared.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM poor ground connection caused high-voltage system voltage detection drift

✓
The vehicle intermittently stalled while driving, with multiple warning lights illuminated on the dashboard. A deep scan revealed an ECU–TCU communication fault with DTC B123B17. Inspection found a loose securing bolt at earth point G107 on the left side of the engine bay, causing the ACU (Air Conditioning Control Unit) reference earth to drift and falsely detect high-side overvoltage. Solution: Removed the earth cable, cleaned the contact surface and harness terminal with sandpaper, applied conductive paste, and tightened the bolt to the specified torque of 9-11 N·m. Also inspected and tightened other high-voltage system earth points.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Backed-out pin in Song DM vehicle control unit wiring harness caused false high-voltage monitoring alarm.

✓
will be just the text.</think>The vehicle could not communicate with the Vehicle Control Unit (VCU). The air conditioning system logged DTC B123B17 (high pressure side overvoltage). Inspected the G09 connector below the instrument panel and found pins 14 and 15 (CAN-H/L) had backed out, preventing the ACU from receiving the high voltage system data correctly. The controller received incorrect voltage signals, triggering protection mode. Re-seated the backed-out pins in the wiring harness, replaced the damaged connector terminals, checked the condition of fuse F4/9 (VCU power), and used VDS to perform ‘Network Configuration’ and ‘Air Conditioning System Initialisation’. This cleared the fault.
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. Sources: [1]