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B1773

On BYD new energy vehicles, DTC B1773 indicates a Cell Supervision Circuit (CSC) fault — Seal U

Safety System

On BYD new energy vehicles, DTC B1773 indicates a Cell Supervision Circuit (CSC) fault.

This represents a Battery Management System (BMS) subsystem communication or sampling abnormality, rather than the seat belt pretensioner issue in the original description (the latter belongs to the SRS system, indicating a possible mix-up in fault code definitions).

The CSC module continuously monitors key parameters within the power battery module, including individual cell voltage and temperature, and communicates with the main BMS via the internal CAN bus.

The BMS sets DTC B1773 when it fails to detect a response signal from a CSC within the specified timeframe (communication timeout), receives out-of-range data (e.g., a -40°C open-circuit temperature reading), or detects a CSC address conflict or duplication.

This fault activates the system safety protection strategy, limiting vehicle power output, disabling pure EV driving, or interrupting the charge/discharge process.

It severely compromises power battery thermal management and safety monitoring functions and requires immediate repair.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Collector wiring harness connector oxidized, corroded, poor contact, or terminal backed out (common after water ingress due to battery pack seal failure)
  • 2Internal short circuit in the battery information collector (CSC module), cold or broken PCB solder joints, or damaged sampling chip.
  • 3Collector low-voltage power supply circuit fault (blown power supply fuse or open circuit causing loss of 12V power supply)
  • 4Collector CAN communication line fault (resistance between CAN-H and CAN-L deviates from the 60Ω standard value, line open or short circuit)
  • 5Incorrect collector address coding configuration, or failure to perform the Address Learning procedure after replacing components, resulting in a system identification conflict.
What To Do
  • 1
    Use the VDS diagnostic tool to access the BMS system and read the complete fault codes and live data stream. Confirm the specific collector number (Group X CSC) and fault type (communication fault/sampling abnormality).
  • 2
    Perform the high-voltage safety power-down procedure: disconnect the low-voltage battery negative terminal, remove the high-voltage service disconnect (MSD), wait at least 5 minutes for the high-voltage system to fully power down, and perform an insulation test.
  • 3
    Measure the fault collector low-voltage supply voltage (standard value: 12V ± 0.5V) and CAN bus voltage (CAN-H: approx. 2.5–3.5V, CAN-L: approx. 1.5–2.5V). Measure the CAN line terminating resistance (standard value: approx. 60Ω; excessive deviation indicates a wiring fault).
  • 4
    Remove the power battery pack upper cover. Visually inspect the collector connector for oxidation or water ingress, the wiring harness pins for back-out or looseness, and the wiring harness retaining clips for breakage. Clean any oxidation from the connector, use the special tool to repair backed-out pins, and replace the low-voltage wiring harness if necessary.
  • 5
    Remove the faulty collector and measure the resistance of its temperature sampling NTC (compare against the standard temperature-resistance curve). Check the PCB solder joints for cold or broken joints. If the collector has internal damage, replace it with a CSC module of the same model.
  • 6
    If installing a new collector or suspecting an address conflict, use the diagnostic tool to perform the 'Battery Information Collector Address Configuration/Learning' (Address Configuration) and 'Capacity Learning/Reset' (Capacity Reset) operations to ensure each collector address matches its physical location.
  • 7
    Reassemble the battery pack, check sealing ring integrity, and tighten bolts to the specified torque. Restore high-voltage connections, clear fault codes, and perform a road test to verify normal EV mode driving and AC/DC charging functions.
Real-World Cases
BYD DTC AI Analysis

Song Pro DM battery information collection module communication fault (connector oxidation)

✓
Symptoms: Dashboard displayed "Check Powertrain", EV mode unavailable, engine forced to start. Scanner read DTCs B1773 (Battery Information Collector fault) and P1A3400 (Battery Management System fault). Diagnosis: VDS showed CSC communication fault on string 5; removed battery pack detection harness and measured collector CAN line resistance >120 Ω (normal 60 Ω); inspection found corroded collector connector with backed-out terminal. Solution: Cleaned connector oxides, repaired backed-out terminal, and reconnected; resistance normalized. Cleared DTCs; EV mode restored. Check battery pack sealing to prevent moisture ingress.
Original source ↗
BYD DTC AI Analysis

Qin Pro DM temperature sampling abnormality (cold solder joint on chip)

✓
Symptoms: Sudden power loss while driving; dashboard displayed "Powertrain Fault." Read DTC B1773 and multiple temperature sampling fault codes. Diagnosis: Live data showed Module 3 acquisition unit reporting all temperatures at -40°C (open circuit condition). Removed the battery pack and located the Module 3 CSC. NTC resistance measured normal, but the acquisition board had cold solder joints on the temperature sampling chip pins. Resolution: Replaced the Module 3 battery information collection board and reflashed the configuration parameters. Performed "Battery Information Collector Address Configuration" and "Capacity Learning." Fault cleared.
Original source ↗
BYD DTC AI Analysis

Tang DM charging system fault (internal short circuit in sampling unit)

✓
Symptoms: Vehicle failed to charge (AC and DC both inoperative), instrument cluster displayed 'Charging System Fault'. Retrieved DTC B1773 and multiple BMS communication timeout faults. Diagnosis: OBC-to-BMS communication normal. Inspected the battery pack and found Group 8 CSC had lost communication with the master BMS. Measured the module supply voltage at 0V (normal 12V); the power fuse had blown. Replaced the fuse but it blew again immediately—internal short in the CSC. Resolution: Replaced the Group 8 CSC module and power fuse. Found impact damage to the battery pack underside had deformed the CSC housing and caused a short. Repaired the underbody shield to ensure sealing integrity.
Original source ↗
BYD DTC AI Analysis

e5 EV data collector address configuration error

✓
Symptoms: Dashboard displayed powertrain fault warning with speed limit active; DTC B1773 would not clear. Diagnosis: Scan tool showed "Group 6 acquisition module address conflict" and "communication fault". Inspection found a previous battery module replacement at a non-authorised workshop. The replacement acquisition module had incorrect address coding; hardware functioned correctly but the wrong address prevented BMS recognition. Resolution: Used a dedicated scan tool to execute "battery acquisition module address learning", reconfiguring each acquisition module address to its physical location. Performed "system reset" and "insulation test"; fault cleared.
Original source ↗
BYD DTC AI Analysis

Song DM intermittent fault (poor harness contact)

✓
Symptoms: Intermittent powertrain fault with EV mode unavailable when active; fault sometimes cleared after restart; frequency gradually increasing; historic DTC B1773 present. Diagnosis: Performed wiring harness wiggle test. Fault reoccurred when wiggling low-voltage wiring near the high-voltage battery pack. Inspection revealed broken retaining clip for the low-voltage loom inside the battery pack, and loose Module 2 cell monitoring connector with slightly burnt pins. Repair: Replaced wiring retaining clip and secured the loom. Cleaned and polished the Module 2 cell monitoring connector pins, applied conductive paste, reconnected the plug and secured it with cable ties. Road-tested 50 km; fault did not reoccur.
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.