AU
B123A16

This DTC indicates an undervoltage fault in the low-pressure side control circuit of the BYD new energy vehicle thermal management system — Seal U

Thermal Management System

This DTC indicates an undervoltage fault in the low-pressure side control circuit of the BYD new energy vehicle thermal management system.

Specifically, the 12V low-voltage supply to the air conditioning system (electric compressor, PTC heater, or thermal management control module) drops below the normal operating threshold (generally below 9.5V or 80% of the system set value), or the A/C low-pressure sensor reference or signal voltage is abnormal.

In models such as the Qin EV300 and Qin PRO DM, this fault causes insufficient power supply to the Thermal Management Module (TMM) or Air Conditioning Control Unit (ACU).

This shortage can prevent the electric compressor from starting, limit PTC heater output, and disable battery cooling and heating functions.

Although the vehicle remains drivable, A/C cooling and heating performance drops significantly.

In extreme cases, high-voltage battery thermal management fails and triggers power limitation.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Insufficient 12V low-voltage battery charge, battery aging, or a charging system fault causing a voltage drop at the thermal management module power supply.
  • 2Loose low-voltage connector, oxidized pins, or backed-out pins at the thermal management control module (TMM), air conditioning controller, or electric compressor, causing excessive contact resistance.
  • 3A/C low-pressure sensor (located at the compressor suction port or liquid line) circuit short to ground, open circuit, or sensor internal short circuit
  • 4High temperatures or wear damage the engine compartment or chassis wiring harness insulation, causing intermittent short circuits or voltage drops.
  • 5Mismatched rated current of related fuses (such as compressor control fuse and TMM power supply fuse), burnt fuse base, or burnt relay contacts.
What To Do
  • 1
    Use the VDS diagnostic tool to read the DTC freeze frame, confirm if B123A16 is a current fault, and record the ambient temperature, battery SOC, and low-voltage side voltage at the time of the fault.
  • 2
    Measure the 12V battery static voltage (should be ≥12.4V) and dynamic charging voltage (should be ≥13.8V). Check the battery state of health (SOH).
  • 3
    Check the power supply fuse (usually 30A or 40A), relay, and connector for the thermal management control module (located in the front compartment or passenger compartment). Measure the voltage between the module B+ terminal and ground to verify it is within the 11-14V range.
  • 4
    Check the electric compressor low-voltage connector (4-pin or 6-pin, including 12V+, GND, CANH, CANL, etc.), measure the 12V supply voltage, and check the pins for oxidation or backing out.
  • 5
    If equipped with a PTC heater, check the PTC controller low-voltage supply voltage; check the A/C low-pressure sensor 5V reference voltage and signal feedback voltage (normal: 0.5-4.5V).
  • 6
    Perform wiring harness wiggle and vibration tests, focusing on harness wear in front compartment high-temperature areas and firewall pass-through holes. Repair poor connections or replace the wiring harness.
  • 7
    Clear the fault code, execute the air conditioning system self-learning procedure (such as the compressor run-in procedure), perform a cooling/heating function test, and verify the 'low-pressure side voltage' in the data stream returns to normal.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song MAX seat sensor circuit fault

✓
Symptoms: Instrument cluster displayed seat system fault light, driver’s seat heating inoperative, and airbag warning light illuminated intermittently. Diagnosis: Scanned with VDS diagnostic tool and retrieved DTC B123A16 (driver seat position sensor circuit fault). Inspected wiring harness beneath seat and found loose connector. Measured sensor supply voltage: 5V (normal). Signal line voltage fluctuated between 0.2V and 2.8V. Resolution: Resecured seat wiring harness connector, cleaned pin oxides and applied conductive grease. Fault cleared.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro seat memory function failure

✓
Symptoms: On models with seat memory, the driver seat memory position would not store and the memory button did not respond when pressed. Diagnosis: Retrieved DTC B123A16 (driver seat position sensor signal abnormal). Disassembled the seat and found the position sensor track deformed, causing resistance drift. Normal resistance range is 500–4500 ohms; actual measurement showed an open circuit at specific positions. Resolution: Replaced the driver seat position sensor assembly, recalibrated the seat memory module, and performed the 'Seat Position Learning' procedure.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Tang DM seat heating and ventilation malfunction

✓
Symptoms: Driver's seat heating and ventilation failed simultaneously while seat adjustment remained functional. Diagnosis: VDS retrieved DTC B123A16 accompanied by U0155 (instrument cluster communication fault). Inspection confirmed the seat control module (SCM) had normal power supply, but the CAN-H circuit was shorted to ground. Tracing section by section revealed that interference between the seat rail and the body had chafed through the wiring harness. Resolution: Repaired the damaged CAN wiring, rerouted the harness to clear the seat rail travel path, and secured it with cable ties.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Han EV seat welcome function inoperative

✓
Symptoms: After turning off the ignition and opening the door, the automatic seat retraction function failed. When entering the vehicle and starting up, the seat did not return to the memory position. Diagnosis: A full vehicle scan revealed DTC B123A16 (driver's seat horizontal position sensor fault) stored in the BCM. Live data showed the sensor signal voltage dropping abruptly in steps during seat movement, indicating internal poor contact within the sensor. Resolution: Replaced the complete seat track frame assembly (including the position sensor). Programming and calibration resolved the fault.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song Plus DM-i seat rattle with warning light

✓
Symptoms: Clicking noise from the driver's seat during fore-aft adjustment, and the dash occasionally reports a seat system fault. Diagnosis: Retrieved DTC B123A16. Disassembled the seat and found excessive metal debris at the meshing point between the position sensor gear and the slide rail rack, causing erratic sensor signals. Measured sensor power and ground as normal, but the signal output waveform was erratic. Resolution: Cleaned the slide rails and sensor gear, applied grease, and readjusted the sensor mounting position.
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]