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 — Seal U

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.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1A damaged internal drive circuit or speed detection circuit in the compressor controller (IPM module) prevents interpretation of the speed sensor Hall signal.
  • 2Compressor internal Hall speed sensor failure (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 the compressor (pin corrosion, terminal back-out, water seal failure)
  • 4Unbalanced compressor motor three-phase windings (resistance deviation >10%) or local inter-turn short circuit causes actual speed to deviate excessively from target speed.
  • 5Outdated air conditioning control module (AC ECU) software version with a control strategy bug falsely reporting 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 DTCs and freeze frame data. Verify B173B is a current fault (Active) rather than a history fault (History). Record the compressor speed, current, and voltage data at the time the fault occurred.
  • 2
    Visually inspect the electric compressor exterior and the high- and low-pressure pipe connections. Confirm no refrigerant leakage or physical damage. Check the controller heat sink for discoloration caused by overheating.
  • 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 resistance of the three-phase compressor motor windings (U-V, V-W, W-U). Normal resistance is 1.0-3.0Ω (refer to the vehicle repair manual for specific values). The three phases must balance (deviation <10%). If 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, and frequency varies with speed (30-90Hz corresponds to 900-2700rpm). If the signal is missing or the waveform is distorted, check the sensor and wiring.
  • 6
    Check the fastening condition of the compressor controller-to-body ground point (usually located inside the fender or on the longitudinal beam). Clean the oxidation layer from the ground point using sandpaper, apply conductive paste, and tighten to the specified torque (usually 8-10 N·m).
  • 7
    If all circuit measurements are normal, attempt a software update on the air conditioning control module (check BYD TPI technical bulletins for relevant update procedures) to rule out a false software fault.
  • 8
    Replace the faulty component: First, replace the compressor controller (some models allow separate replacement; others require replacing the compressor assembly). After replacement, perform the air conditioning system initialization and matching (self-learning procedure) to enable the controller to recognize the compressor parameters.
  • 9
    Use dedicated equipment to evacuate the system (vacuum level <-0.1 MPa, hold for 30 minutes). Charge the specified amount of R134a refrigerant (usually 600–800 g) and POE compressor oil (usually 150–200 ml, deducting the amount of residual old oil). Perform a pressure holding 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 0.15-0.25 MPa, high pressure 1.2-1.6 MPa), and confirm fault resolution.
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 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.