AU
B110F09

DTC B110F09 indicates a High Voltage Interlock Loop (HVIL) fault, contrary to some materials that incorrectly label it as a "PM2 — Seal U

Thermal Management System

DTC B110F09 indicates a High Voltage Interlock Loop (HVIL) fault, contrary to some materials that incorrectly label it as a "PM2.5 rapid tester solenoid valve failure".

The HVIL serves as a critical safety protection mechanism on BYD new energy vehicles.

The system triggers this code when it detects compromised high-voltage circuit integrity, such as a loose connector, an open wiring harness, or an abnormal service disconnect switch.

Upon logging this fault, the BMS immediately opens the high-voltage relays and disables high-voltage power-up to prevent electric shock.

As a hard safety fault, it prevents the vehicle from entering READY mode or causes a sudden loss of power while driving.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Poor contact, broken locking clip, or oxidized pins at the battery pack low-voltage wiring harness connector (BMS 32-pin connector), causing abnormal resistance in the interlock circuit.
  • 2Burnt internal interlock pins, deformed spring contacts, or water ingress corrosion in the Manual Service Disconnect (MSD), causing an open circuit.
  • 3The interlock wiring harness for the DC charging port or high-voltage power distribution box interferes with and chafes against the vehicle body. Damaged insulation causes an intermittent short circuit or open circuit.
  • 4After accident repair or maintenance, high-voltage wiring harness connector not installed to specified torque, and secondary locking mechanism not fully engaged.
  • 5Poor sealing of interlock circuit connectors causes pin oxidation and corrosion after prolonged water exposure, increasing contact resistance.
What To Do
  • 1
    Use VDS2000 or a dedicated diagnostic tool to read all fault codes. Confirm B110F09 and accompanying high-voltage interlock-related fault codes (e.g., P1A6000).
  • 2
    Check the Manual Service Disconnect (MSD) installation status. Measure the resistance between the interlock pins (normal: continuity, less than 10 Ω). Replace the MSD assembly if necessary.
  • 3
    Verify the locking mechanism of the battery pack front-end BMS low-voltage wiring harness connector (32-pin orange connector) is intact. Confirm the secondary lock clicks fully into place. Clean any oxidation from the pins and apply conductive grease.
  • 4
    Inspect the interlock wiring harness along the interlock circuit for high-voltage components, including the high-voltage distribution box, DC charging port, and PTC. Focus on interference points with metal body brackets. Repair any damaged wiring harness and install corrugated conduit.
  • 5
    Use a multimeter to measure the continuity and insulation to ground of the entire interlock circuit. Verify there are no open circuits, short circuits, or abnormal resistance.
  • 6
    Disconnect and reconnect all interlock-related connectors. Verify full insertion and secure locking. Secure any loose plugs with cable ties.
  • 7
    Reinstall all components, install the MSD, connect the 12V battery, and clear the fault codes.
  • 8
    Perform the 'High Voltage System Self-check' and 'Insulation Test' procedures. After confirming no electrical leakage risk, attempt to power on the vehicle and verify the READY status and EV/HEV mode functions.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Plus DM-i battery pack low-voltage connector poor contact

✓
Symptoms: Vehicle wouldn't power up, the instrument cluster displayed 'Check Powertrain', and the system failed to enter READY mode. Diagnosis: Retrieved DTC B110F09 (High Voltage Interlock Loop fault). Inspected the battery pack BMS 32-pin connector and found the locking tab broken, with interlock pins oxidised and loose. Measured an open circuit across the interlock loop. Solution: Cleaned oxidation from the pins and applied conductive grease. Reseated the connector and secured it with cable ties. Cleared the DTCs. High voltage system self-check passed. Fault resolved.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Song Pro DM maintenance switch (MSD) internal burning

✓
Symptoms: Sudden loss of power while driving, vehicle entered limp mode, instrument cluster displayed 'Powertrain System Fault', EV mode unavailable. Diagnosis: Read DTCs B110F09 and P1A6000. Measured MSD interlock pin resistance: infinite. Disassembled unit and found internal interlock pin burn damage and contact spring deformation. Resolution: Replaced service switch assembly (Part No.: 6A-8114120). Checked high-voltage wiring harness, found no abnormalities, then reassembled. Cleared DTCs and tested. EV/HEV modes returned to normal.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Han EV fast charging port interlock harness damaged and shorted

✓
Symptoms: DC fast charging repeatedly disconnects, automatically interrupting after 5–10 minutes. The instrument cluster displays 'Charging System Fault'. AC slow charging operates normally. Diagnosis: Read DTC B110F09 accompanied by U015587. Inspected the wiring harness behind the DC fast charging inlet and found interference with the body bracket. The interlock wiring harness insulation had chafed through, causing an intermittent short circuit. Measured signal wire resistance to ground fluctuating between 0–50 Ω. Solution: Repaired the damaged wiring harness, rewrapped it with insulating tape and heat shrink tubing, rerouted the harness and fitted corrugated tubing, then recalibrated the charging system. Fault resolved.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

High-voltage connector not locked after Tang DM-i accident repair

✓
Symptoms: Vehicle would not start after sitting overnight. Dashboard displayed "Check power battery". Vehicle previously experienced an underbody impact; the battery pack shield was repaired at a workshop. Diagnosis: Retrieved DTC B110F09. Inspected the high-voltage harness connector at the front of the battery pack. Found connector not fully inserted, locking mechanism not engaged, interlock circuit open. Resolution: Disconnected the MSD and waited 5 minutes for discharge. Re-seated the high-voltage harness connector and confirmed secondary lock engaged. Performed high-voltage system self-test and insulation check. Cleared DTC, road test normal.
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]