AU
U2ABC16

DTC U2ABC16 indicates the electric air conditioning compressor high-voltage load circuit detects an input voltage below the normal operating threshold (typically below 420VDC) — Seal U

Thermal Management System

DTC U2ABC16 indicates the electric air conditioning compressor high-voltage load circuit detects an input voltage below the normal operating threshold (typically below 420VDC).

In the 2019 BYD Qin EV thermal management system, the high-voltage battery pack (nominally 500V+) powers the electric compressor through the high-voltage power distribution box.

This fault indicates the compressor controller detects insufficient bus voltage to maintain normal compressor startup or operation during self-check or running.

This fault triggers a protective compressor shutdown, causing air conditioning cooling failure.

In extreme cases, it may affect battery thermal management (liquid cooling system), but typically does not directly cause a vehicle breakdown.

The root cause involves an abnormality in the high-voltage power supply circuit, Battery Management System (BMS) voltage monitoring, or the compressor controller internal voltage sampling circuit.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Traction battery pack SOC too low or cell voltage difference too large: When total battery pack voltage falls below 420V or individual cell voltage falls below 3.0V, the BMS limits power output, resulting in insufficient power supply to the compressor.
  • 2High-voltage distribution box compressor contactor welded or poor contact: Oxidized or burned contactor contacts increase contact resistance, causing an excessive voltage drop during compressor startup.
  • 3Electric compressor controller (IPM module) internal fault: Controller internal bus voltage sampling circuit fault, precharge circuit failure, or IGBT drive fault causing a false low-voltage reading.
  • 4Poor contact or terminal back-out in the high-voltage wiring harness: Loose connections, backed-out terminals, or burn damage in the high-voltage connectors between the battery pack and the high-voltage distribution box, or between the high-voltage distribution box and the compressor, causing an abnormal circuit voltage drop.
  • 5Precharge resistor or precharge relay fault: An abnormal precharge circuit causes insufficient charging of the compressor controller bus capacitor, triggering a low voltage fault during the power-on check.
What To Do
  • 1
    Safety preparation: Wear insulated gloves, disconnect the low-voltage battery negative terminal, wait 5 minutes, confirm the high-voltage system is de-energized (voltage <60V), and hang a warning sign.
  • 2
    Read freeze frame: Use VDS or ED400 to read the bus voltage, battery SOC, and minimum cell voltage at the time of the fault to confirm whether the low voltage is actual or a sampling fault.
  • 3
    Check the traction battery: Measure the total battery pack voltage and check the cell voltage difference in the BMS data stream. If the lowest cell voltage is <3.0V or the voltage difference is >300mV, resolve the battery fault first.
  • 4
    Inspect the high-voltage distribution box: measure the voltage drop across the compressor contactor input and output terminals (normal: <1V), check the contactor drive signal and inspect for welded contacts, and replace the high-voltage distribution box if necessary.
  • 5
    Check the compressor high-voltage wiring harness: Disconnect the compressor high-voltage connector, check for burnt or backed-out pins, and measure harness continuity and insulation resistance (>500MΩ).
  • 6
    Check the compressor controller: Measure the voltage at the controller high-voltage input terminal and compare it to the BMS display value. If the deviation is >10V, replace the controller. Check the controller low-voltage power supply and CAN communication.
  • 7
    Precharge circuit test: Use an oscilloscope to capture the bus voltage rise curve at power-up. If the precharge time is >3 seconds or the voltage rises slowly, check the precharge relay and resistor (Qin EV typically 30Ω/40W).
  • 8
    Verify repair: Clear the fault code, power on the vehicle, and observe the bus voltage at compressor startup. Confirm no fault codes return and the air conditioning cools normally.
Real-World Cases
BYD DTC AI Analysis

Qin EV air conditioning suddenly stopped working while driving

✓
Driving normally when the AC suddenly blew hot air. No warning lights on the dash, but the AC panel was flashing. Scanned DTC U2ABC16. Freeze frame showed bus voltage at 398V and SOC at 35% when the fault occurred. Further inspection found cell 12 in the battery pack at only 2.8V while the remaining cells read 3.3V—a 500mV difference. This undervoltage caused the BMS to limit power output, and the drop in bus voltage triggered compressor protection. Replaced the module and performed a balancing charge. Fault resolved.
BYD DTC AI Analysis

High-voltage distribution box contactor burn damage caused intermittent fault

✓
Intermittent AC cooling failure; recovered after key cycle. DTC U2ABC16 logged intermittently. Static voltage at the compressor HV terminal: 500V (normal), but dropped to 380V instantly when AC engaged. Removed the HV distribution box and found severe burning on the compressor positive contactor contacts. Contact resistance: 2.3Ω (normal <0.1Ω). Replaced the HV distribution box assembly (part number: KAD-2152410). Loaded voltage now stays above 495V. Fault has not returned.
BYD DTC AI Analysis

Compressor controller pre-charge circuit failure

✓
Air conditioning shut off automatically within 10 seconds of power-on, logging DTC U2ABC16. Compressor controller bus voltage measured only 280 V at power-on, far below the normal pre-charge target (>450 V). Pre-charge relay tested normal, but the pre-charge resistor read infinite resistance (on the Qin EV, this resistor sits inside the battery pack). Disassembled the battery pack and replaced the pre-charge resistor (30 Ω). Inspection also revealed capacitor aging inside the compressor controller; replaced the compressor controller as well. Fault resolved.
BYD DTC AI Analysis

High voltage harness pin backed out causing intermittent connection.

✓
The air conditioning cut out frequently on rough roads but worked normally on smooth surfaces. Inspection revealed the compressor power pin in the high-voltage connector (large orange plug) between the battery pack and high-voltage distribution box had backed out approximately 1 mm, with burn marks on the socket. Driving vibration caused poor contact, instantly increasing line resistance, and the compressor controller detected the voltage drop. Replaced the high-voltage wiring harness connector, applied conductive paste, and tightened the plug to resolve the fault.
BYD DTC AI Analysis

BMS voltage sampling drift false positive

✓
At SOC 100%, turning on the AC immediately set DTC U2ABC16. Battery pack voltage measured 512V and compressor-side voltage 508V — both normal. Scan tool showed BMS compressor bus voltage at 395V. Fault traced to internal BMS voltage sampling circuit or voltage divider resistor, causing incorrect voltage data to the compressor controller. Replaced the battery management unit (BMS main unit, located inside the battery pack or cabin depending on configuration), flashed BMS software. Fault cleared.
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.