AU
U2ABB17

DTC U2ABB17 indicates the electric compressor controller (PCU) detects the input DC high voltage exceeds the permitted operating threshold (typically 110%-120% of the rated voltage) — Seal U

Thermal Management System

DTC U2ABB17 indicates the electric compressor controller (PCU) detects the input DC high voltage exceeds the permitted operating threshold (typically 110%-120% of the rated voltage).

For example, on the 2019 Qin EV, the system triggers this fault when the high-voltage battery pack voltage exceeds approximately 420-450V or the compressor internal bus voltage reading exceeds the safe range.

This fault acts as a high-voltage safety protection mechanism for the thermal management system.

Upon detecting abnormally high voltage, the compressor actively disconnects the high-voltage relay to protect the IGBT power module and motor insulation.

Actual battery pack overvoltage, voltage sampling circuit drift, high-voltage interlock circuit faults, or control software logic errors can cause this fault.

Determine whether the condition is a genuine voltage abnormality or a signal detection fault.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Traction battery voltage too high at the end of fast charging: During DC fast charging to above 90% SOC, the battery pack voltage approaches the full-charge voltage (about 420V-450V). If the BMS fails to correctly limit the charging voltage or the compressor overvoltage threshold setting is too low, the system triggers protection.
  • 2Electric compressor controller (PCU) internal voltage sampling circuit fault: Voltage divider resistor aging, ADC drift, or filter capacitor failure causes the sampled value to exceed the actual value.
  • 3Poor contact in the high-voltage wiring harness: An intermittent, burnt, or loose connection at the compressor high-voltage connector generates transient overvoltage spikes during load changes (back EMF when disconnecting an inductive load).
  • 4Abnormal communication between the BMS and compressor controller: CAN bus interference or incorrect voltage data from the BMS causes the compressor to falsely detect an overvoltage condition.
  • 5Abnormal regenerative braking energy recovery: Under specific operating conditions (such as high regeneration on long descents), the battery pack voltage momentarily exceeds the allowable compressor input range.
What To Do
  • 1
    Read freeze frame data: Use the VDS2000/3000 diagnostic tool to read detailed data from the moment the fault occurred, including compressor high-side voltage, battery pack total voltage, SOC, current direction, and compressor operating status, to confirm a genuine overvoltage condition.
  • 2
    Measure the actual high-voltage system voltage: Use an insulation tester and a multimeter to measure the total traction battery voltage. Compare this value with the BMS voltage and the compressor reported voltage in the scan tool data stream. A deviation exceeding 5V indicates a fault in the sampling circuit.
  • 3
    Check the high-voltage connection status: Disconnect the service disconnect and wear insulated gloves. Inspect the compressor high-voltage connector (usually located on the right side of the front compartment) for burning, backed-out terminals, or water ingress. Measure the continuity of the high-voltage interlock circuit.
  • 4
    Low-voltage circuit diagnosis: Check the compressor controller 12V supply voltage (13.8-14.2V, ignition ON) and the tightness of ground points (G201, etc.). Measure CAN-H and CAN-L resistance (approximately 60Ω) and waveforms.
  • 5
    Insulation resistance test: Use a megohmmeter to measure the insulation resistance between the compressor high-voltage positive and ground, and between the high-voltage negative and ground. The resistance must be greater than 20MΩ to rule out voltage abnormalities caused by motor insulation faults.
  • 6
    Software version check and update: Verify the software versions of the BMS, thermal management controller, and compressor controller. Compare them against the Technical Service Bulletin (TSB) and execute the latest software flash procedure to correct the voltage threshold calibration.
  • 7
    Component replacement verification: If the above checks are normal, swap the compressor controller or compressor assembly with one from a known good vehicle of the same model to verify if the fault transfers. Confirm the faulty component and replace it.
Real-World Cases
BYD DTC AI Analysis

Air Conditioning Failure After Fast Charging

✓
A 2019 Qin EV was fast-charged to 98% on a 120kW DC charger during winter in the north. The AC then failed to cool. DTC U2ABB17 logged. Freeze frame data showed the compressor reading 452V while the actual battery pack voltage was 438V. Replaced the compressor controller, but the fault remained. Eventually traced this to outdated BMS software (Ver 3.2) with a faulty voltage sampling compensation algorithm at high SOC. Flashed the BMS to Ver 4.1. The compressor then received correct voltage data and the fault cleared.
BYD DTC AI Analysis

Loose connection in compressor high-voltage harness triggered overvoltage protection

✓
While driving normally, the air conditioning suddenly stopped working and the dashboard displayed "Thermal Management System Fault". Found intermittent DTC U2ABB17. Checked the compressor high-voltage connector (BYD standard blue plug); the internal HV+ terminal had backed out approximately 2mm, increasing contact resistance to 0.8Ω (normal <0.1Ω). Arcing and voltage spikes during compressor start/stop triggered overvoltage protection. Re-crimped the terminal and applied high-voltage silicone grease; contact resistance dropped to 0.05Ω and the fault cleared.
BYD DTC AI Analysis

Voltage sampling circuit resistor aging

✓
After the vehicle sat overnight, DTC U2ABB17 set immediately on first start and the air conditioning would not turn on. Measured battery pack voltage at 386V (normal), but live data showed compressor detected voltage at 468V, indicating a sampling circuit fault. Removed the compressor controller (located on top of the compressor) and measured the high-voltage sampling voltage divider resistor network. Found R15 (2MΩ/1%) had drifted to 2.4MΩ, causing incorrect voltage division. Replaced the precision resistor; sampling value returned to normal (387V), fault resolved.
BYD DTC AI Analysis

High regeneration during extended downhill driving caused momentary overvoltage

✓
After a 10 km long downhill mountain drive, the air conditioning stopped working and the 'Power System Fault' warning light illuminated. DTCs read U2ABB17 and related BMS overvoltage codes. Data analysis showed the battery pack voltage spiked from 410 V to 463 V at the moment of failure—high regenerative braking and high internal resistance from cold ambient temperatures caused the spike. Inspection of the BMS and compressor software revealed the compressor overvoltage protection threshold was set at 460 V, while the battery pack allowable charge voltage is 465 V. The threshold was too sensitive. Upgraded the thermal management controller software, raising the overvoltage protection threshold to 475 V and optimizing the delay strategy. Issue resolved.
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.