AU
B2A6700

This DTC indicates the Electric A/C Compressor (EAC) fails to establish normal speed or pressure after multiple consecutive start attempts following a start command — Atto 8

Thermal Management System

This DTC indicates the Electric A/C Compressor (EAC) fails to establish normal speed or pressure after multiple consecutive start attempts following a start command.

The system logs a start failure fault.

The root cause is the compressor controller failing to provide a valid speed signal within the specified time (typically 2-3 seconds) or failing to build torque.

On plug-in hybrid (DM) models, an electric compressor failure prompts the system to start the engine to drive the mechanical compressor (Belt-driven Compressor) as a backup cooling solution, triggering DTC B2A6700.

This fault causes a complete loss of A/C cooling capacity, which is particularly severe in pure electric mode.

Additionally, because the BYD thermal management system couples the battery chiller (Chiller) with the A/C system, a prolonged fault can cause power battery thermal management failure, triggering power limits or even high-voltage interlock disconnection.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Electric compressor high-voltage power supply fault: Causes include a blown compressor fuse in the high-voltage distribution box, poor contact in the high-voltage interlock loop (HVIL), burnt main negative/main positive contactors, or a damaged IPM module on the compressor's internal motor drive board, preventing the compressor from receiving sufficient high-voltage DC power (typically 320V-750V).
  • 2Compressor mechanical seizure or motor fault: Internal scroll plate wear and seizure, motor permanent magnet demagnetization, bearing seizure, or degraded refrigerant oil causes excessive start-up torque, which triggers controller overcurrent protection and causes start-up failure.
  • 3Low-voltage control communication fault: LIN bus or CAN bus communication interruption (between the BMS, VCU, and compressor controller), abnormal wake-up signal, or enable signal circuit open or short to ground, preventing the compressor controller from receiving the correct speed command.
  • 4Refrigerant pressure protective shutdown: Severe system refrigerant shortage (leakage) causes excessively low pressure on the low-pressure side, or excessive refrigerant/poor condenser heat dissipation causes excessively high pressure on the high-pressure side, triggering pressure switch protection to prevent compressor startup.
  • 5Thermal Management System (TMS) controller or air conditioning controller software bug: an internal controller program logic error falsely reports a startup failure; or the algorithm threshold for deviation between the compressor target speed and actual control value is too sensitive.
What To Do
  • 1
    Diagnostic tool reading and freeze frame analysis: Use VDS2000 or Launch X431 to read all fault codes. Check for accompanying high-voltage interlock (P0A0D, P0A0E) or insulation (P1A00 series) faults. Record the ambient temperature, battery SOC, compressor target speed, and actual speed data streams at the time of the fault.
  • 2
    High-voltage safety check and insulation test: Disconnect the manual service disconnect (MSD) and wait 5 minutes for high-voltage discharge. Use a megohmmeter to measure the electric compressor high-voltage harness insulation resistance to ground (standard >20MΩ). Inspect the high-voltage connectors for burning or backed-out pins. Measure the high-voltage interlock circuit continuity.
  • 3
    Low-voltage circuit and communication check: Disconnect the compressor low-voltage connector and measure the pins: constant power (B+) should be 12V, ground (GND) <0.1Ω, LIN line voltage approximately 9-11V (dynamic), and CAN-H/CAN-L voltage to ground approximately 2.5V. Use an oscilloscope to check the LIN bus waveform for distortion.
  • 4
    Refrigerant pressure and system sealing check: Connect a manifold gauge set and check the static pressure (R134a at approx. 25°C: 0.6-0.8 MPa). If the pressure is too low, evacuate the system using a vacuum pump and hold the vacuum for 30 minutes. Use an electronic leak detector to locate leaks (commonly found at the compressor shaft seal and high/low-pressure ports).
  • 5
    Compressor operation test and current monitoring: Restore the high-voltage connection. Enter the scan tool actuator test mode and gradually increase the compressor speed (1000rpm→3000rpm→6000rpm). Monitor the high-voltage current using a clamp meter. Normal start-up current must be <15A and rise steadily. If the current is >40A, immediately cut the power. If the current is >40A or oscillates, the compressor assembly is faulty.
  • 6
    Controller software update and replacement: If all above checks are normal, update the Thermal Management Controller (TMS) and compressor controller to the latest software version. If the fault persists, replace the electric compressor assembly (Note: After replacement, add the specified ND-OIL8 refrigerant oil or equivalent, and charge the standard amount of refrigerant).
Real-World Cases
BYD DTC AI Analysis

Qin Pro DM: Backed-out HVIL pin causing intermittent no-start

✓
Intermittent A/C not cooling with DTCs B2A6700 and P0A0D (high voltage interlock fault). Found a backed-out pin in the HVIL circuit of the electric compressor high voltage connector; contact failed over bumps. Re-crimped the pin and applied conductive grease. Fixed. Common on 2018–2019 Qin Pro models due to weak retaining clips in early high voltage connector designs.
BYD DTC AI Analysis

IGBT module inside Qin EV300 compressor burnt out, causing complete failure.

✓
At 80,000 km, the AC failed completely. Scan tool showed DTC B2A6700. During actuator testing, the compressor did not respond and high-voltage side current read zero. Disassembled the compressor controller and found the internal IGBT power module burned out with visible burn marks on the driver board. Replaced the electric compressor assembly (part number: BC28-8103010) and refilled with 150 ml PAG 46 compressor oil and 450 g R134a refrigerant per specification. Fault resolved.
BYD DTC AI Analysis

Minor refrigerant leak triggered low-pressure protection, preventing startup

✓
Poor AC cooling in high summer temperatures. DTC B2A6700 logged. Data stream showed compressor stopped within 2 seconds of engagement, low-side pressure dropping below 0.15 MPa. Vacuum hold test and fluorescent leak detection found a minor leak at the battery cooler (Chiller) connection. Repaired the leak, replaced the receiver drier, evacuated and recharged the system. Pressures normalized (low side 0.2–0.3 MPa, high side 1.4–1.6 MPa). Fault resolved.
BYD DTC AI Analysis

LIN bus interference caused compressor controller communication fault.

✓
Aftermarket audio installation caused intermittent AC failure. Stored DTC B2A6700 with multiple LIN slave node communication faults. Found modified amplifier power wiring running parallel to the AC compressor LIN harness, causing electromagnetic interference. Re-routed wiring to separate power and signal lines (>30 cm apart) and installed a ferrite core filter on the compressor LIN line. Restored communication; fault has not reoccurred.
BYD DTC AI Analysis

Outdated thermal management controller software caused false fault detection.

✓
The 2019 Qin Pro EV frequently set DTC B2A6700 in cold conditions (-5°C), but the compressor ran normally. Live data revealed the controller was overly sensitive to compressor speed overshoot. Upgraded the TMS controller software to the latest version (3.00.15 or above) via VDS2000, adjusting the startup failure threshold. The fault cleared. This is BYD's official software optimisation fix.
Other Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 overview →
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.