AU
B173B

DTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor — Atto 3

Safety System

DTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor.

In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit.

The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm).

This fault forces the A/C system into fail-safe mode and stops compressor operation.

Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance.

The instrument cluster A/C warning lamp may also illuminate.

Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.

The B173B fault code on the Atto 3 indicates an abnormal electric A/C compressor speed feedback signal or a communication fault inside the compressor control circuit. Severity is medium because the A/C may stop cooling and the compressor can enter fail-safe mode. Common causes include a faulty compressor controller/IPM speed detection circuit or a failed built-in Hall speed sensor. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1A faulty internal drive circuit or speed detection circuit in the compressor controller (IPM module) prevents interpretation of the speed sensor Hall signal.
  • 2Compressor built-in Hall speed sensor internal fault (damaged sensor chip, demagnetized magnet, increased installation gap due to vibration)
  • 3Open circuit, short to ground, or poor connector contact in the speed feedback wiring harness between the controller and compressor (pin corrosion, backed-out pin, or failed waterproof seal)
  • 4Unbalanced compressor motor three-phase windings (resistance deviation >10%) or local inter-turn short circuit causes excessive deviation between actual and target speeds.
  • 5Outdated air conditioning control module (AC ECU) software. A control strategy bug falsely reports abnormal speed under specific operating conditions.
What To Do
  • 1
    Connect the BYD dedicated diagnostic tool (VDS2000 or X431). Access the air conditioning system to read the complete DTC and freeze frame data. Confirm B173B is a current fault (Active) and not a history fault (History). Record the compressor speed, current, and voltage data at the time of the fault.
  • 2
    Visually inspect the electric compressor exterior and high- and low-pressure pipe connections. Confirm no refrigerant leaks or physical damage. Check the controller heat sink for overheating and discoloration.
  • 3
    Disconnect the compressor low-voltage connector (usually a 12-pin or 8-pin connector). Use a multimeter to measure the controller-side supply voltage (12V±0.5V), ground resistance (<0.1Ω), and CAN line voltage (CAN-H 2.5-3.5V, CAN-L 1.5-2.5V) to rule out basic wiring faults.
  • 4
    Measure the compressor three-phase motor winding resistance (U-V, V-W, W-U). Normal resistance is 1.0-3.0 Ω (refer to the vehicle repair manual for specific values) and the three phases must balance (deviation <10%). If the resistance is abnormal, replace the compressor assembly.
  • 5
    Use an oscilloscope to check the speed feedback signal wires (typically two thin wires, green/white or yellow/black). A normal signal displays a 0-5V square-wave pulse. Frequency varies with speed (30-90Hz corresponds to 900-2700 rpm). If the signal is missing or distorted, check the sensor and wiring.
  • 6
    Check the tightness of the compressor controller-to-body ground point (usually located on the inner fender or frame rail). Clean the oxidation layer from the ground point using sandpaper, apply conductive grease, and tighten to the specified torque (usually 8-10 N·m).
  • 7
    If all circuit measurements are normal, update the air conditioning control module software (check BYD TPI technical bulletins for relevant update procedures) to rule out a false software fault.
  • 8
    Replace the faulty component: Replace the compressor controller first (some models allow separate replacement; others require replacing the complete compressor assembly). After replacement, perform the air conditioning system initialization matching (self-learning procedure) so the controller recognizes the compressor parameters.
  • 9
    Use dedicated equipment to evacuate the system (vacuum level <-0.1MPa, hold for 30 minutes). Charge the specified amount of R134a refrigerant (usually 600-800g) and POE refrigerant oil (usually 150-200ml, deduct residual old oil). Perform a pressure hold test to verify no leaks.
  • 10
    Clear the fault code. Start the vehicle and set the air conditioning to maximum cooling mode. Observe the compressor speed feedback data stream (actual speed should smoothly follow target speed changes). Verify high and low pressures are normal (low pressure 0.15-0.25MPa, high pressure 1.2-1.6MPa) and confirm the fault is resolved.
Real-World Cases
BYD DTC AI Analysis

Scroll plate seal failure in BYD Tang DM electric compressor caused abnormal speed

✓
The A/C cooling performance deteriorated sharply. Low-side pressure was high (0.8 MPa), high-side pressure low (1.2 MPa), low-pressure line icing, and vent temperature near ambient. Stored DTC B173B (abnormal speed feedback). Diagnosis revealed a design defect in the compressor scroll seal, causing high-pressure gas to leak to the low side, resulting in abnormal compressor load and excessive speed fluctuation that triggered the DTC. Disassembled the compressor, replaced the scroll assembly with the manufacturer's upgraded version (with added seal rings), thoroughly cleaned internal debris, filled with the specified POE refrigeration oil, installed the scroll precisely to marked positions, and sealed the rear cover. Recharged refrigerant and tested; vent temperature dropped to 4°C, speed feedback signal stable, fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD EV compressor controller speed detection circuit damaged

✓
The A/C did not cool at all. The compressor refused to start, and no warning lights showed on the instrument panel. A scan tool pulled current fault code B173B. Checked power, ground, and CAN communication to the compressor controller – all normal. Measured the compressor three-phase motor winding resistance: 1.8 Ω and balanced across phases. Scoped the speed feedback signal wire and got no square wave output, pointing to a failed speed detection IC inside the controller. Replaced the electric compressor controller assembly (on some models the entire compressor must be replaced). Updated the A/C control module software to the latest version and ran the system initialization and matching procedure. After a road test, the speed feedback signal returned to normal (live data showed the speed tracking the command), and the A/C blew fully cold again.
BYD DTC AI Analysis

Poor contact in BYD Qin Pro DM compressor speed sensor wiring harness

✓
The A/C system intermittently failed to cool, with the fault occurring frequently on rough roads or in high ambient temperatures. The diagnostic tool showed code B173B as an intermittent fault. Inspection found the compressor speed sensor connector (located on top of the compressor) had pin retraction and slight water ingress with corrosion inside, caused by long-term exposure to engine bay heat and vibration. Contact resistance was unstable, fluctuating between 0.5 and 50 Ω. Repair: Replaced the speed sensor wiring harness assembly (or repaired the connector), re-crimped the pins, applied waterproof silicone grease, and secured the wiring with heat-resistant cable ties to eliminate stress and prevent contact with surrounding metal components. After clearing the fault codes, a continuous 50 km road test (including rough roads and idle conditions) showed no recurrence. The data stream displayed stable speed feedback with no fluctuation.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 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.