AU
B132316

This DTC indicates the 12V operating supply voltage of the thermal management system (specifically the air conditioning control unit or thermal management controller) falls below the 9V threshold — Seal U

Thermal Management System

This DTC indicates the 12V operating supply voltage of the thermal management system (specifically the air conditioning control unit or thermal management controller) falls below the 9V threshold.

In the BYD new energy architecture, the low-voltage system has a nominal voltage of 12V (normal range 10.5V-14.5V).

When the supply voltage drops below 9V, the controller’s internal microprocessor, sensor interface circuits, and power drive module fail to operate stably.

This instability can cause abnormal air conditioning compressor control signals, inaccurate electronic expansion valve actuation, insufficient battery coolant pump speed, or a protective shutdown of the PTC heater.

As a power supply fault, this condition does not directly trigger the high-voltage interlock, but it indirectly affects battery thermal management performance.

In extreme cases, it can trigger battery over-temperature power derating protection.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Aged or deeply discharged 12V low-voltage battery increases internal resistance, causing a sharp voltage drop under load.
  • 2DC-DC converter or high-voltage system fault, unable to convert high voltage into a stable 14V low-voltage power supply.
  • 3Poor contact in the air conditioning controller/TMS module power supply wiring harness, including loose connectors, corroded pins, or fuse holder oxidation causing a voltage drop exceeding 3V.
  • 4Burnt contacts on the thermal management relay in the front compartment power distribution box, causing increased contact resistance.
  • 5Fault in the air conditioning controller internal power supply voltage regulation circuit, such as filter capacitor leakage or a damaged step-down chip.
What To Do
  • 1
    Connect the VDS diagnostic tool to read the freeze frame data. Verify the specific voltage value at the time of the fault and any accompanying fault codes (such as B132317 overvoltage or U-series communication faults).
  • 2
    Measure the low-voltage battery static voltage. If below 12.4V, charge the battery. Start the vehicle and measure the battery dynamic voltage. The normal range is 13.8V - 14.5V. If below 13V, check the DC-DC converter.
  • 3
    Check the thermal management fuses and relays (such as F1/15 and F2/03) in the front compartment power distribution box. Measure the voltage drop across the relay contacts. Replace the relay if the voltage drop exceeds 0.5V.
  • 4
    Disconnect the 32-pin connector from the air conditioning controller (usually located on the right side of the dashboard or left side of the front compartment). Measure the voltage between the B+ pin (usually pin 1 or 32) and ground. If the voltage is normal and the fault code is present, replace the controller. If the voltage is below 9V, inspect and repair the wiring harness.
  • 5
    Check the intermediate connectors along the power supply path (such as connectors GJ301 and GJ302) for backed-out or oxidized pins. If necessary, apply conductive adhesive or replace the wiring harness.
  • 6
    After clearing the fault code, perform a road test. Monitor the 'IG2 power supply voltage' value in the data stream to confirm it remains above 10V while the air conditioning operates at full load.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro EV abnormal DC-DC output causing intermittent air conditioning shutdown

✓
While driving, the A/C stopped cooling suddenly and the dash displayed ‘Thermal Management System Fault’. Read DTCs: B132316 (working power supply undervoltage) accompanied by U015587 (communication fault with A/C controller). Measured the 12V battery at 11.2V and DC-DC converter output at 12.1V (abnormally low). Found the DC-DC high-voltage input connector burned internally, causing excessive contact resistance. Repaired the HV connector and replaced the DC-DC converter. The low-voltage system returned to 14.2V and the fault cleared.
BYD DTC AI AnalysisFrom Chinese market (translated)

Excessive voltage drop due to worn front compartment wiring harness on Qin EV300.

✓
The customer reported intermittent cabin heating during winter. Diagnosis revealed an intermittent DTC B132316. Measured HVAC controller supply voltage: 11.8 V at idle, dropping to 8.5 V when the PTC heater activated. Inspected the left front engine bay wiring harness and found chafing against the body bracket; damaged insulation on the power wire caused partial copper strand breakage, reducing effective cross-sectional area. Repaired the harness and re-secured the routing; loaded voltage recovered to 13.5 V and thermal management returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

False undervoltage due to increased internal resistance from battery aging

✓
2017 BYD Qin EV300. The air conditioning failed to start after sitting overnight, logging DTC B132316. The 12V battery static voltage measured 12.0V, but collapsed to 7.8V on startup. Battery internal resistance tested at 18mΩ (normal <8mΩ). After replacing the battery, cold start voltage held above 11.5V and the fault code did not return. This case demonstrates that a degraded battery can cause voltage collapse under high current load.
BYD DTC AI AnalysisFrom Chinese market (translated)

Oxidised relay contacts in distribution box caused voltage drop

✓
The vehicle had been operating long-term in high-humidity areas. The AC controller frequently logged DTC B132316. Measured the supply voltage drop from the distribution box to the AC controller and found a 2.3V differential between the IG2 relay output and input terminals (normal <0.2V). Disassembled the relay and found the contacts severely oxidised and blackened. Replaced the relevant relay in the distribution box. The voltage drop returned to normal, and the fault 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. Sources: [1]