AU
C002004

DTC C002004 indicates a functional fault in the return pump motor inside the ABS/ESC Hydraulic Control Unit (HCU) — Atto 8

Braking System

DTC C002004 indicates a functional fault in the return pump motor inside the ABS/ESC Hydraulic Control Unit (HCU).

The return pump is a core actuator in the ABS system.

During the anti-lock braking cycle, it pumps brake fluid from the low-pressure side of the wheel cylinder back to the high-pressure side or the reservoir.

The fault falls into two categories: 1) "Unable to run" — Indicates an open or shorted motor winding, completely worn carbon brushes, a burnt commutator, or a damaged motor drive circuit (H-bridge MOSFET or relay) inside the ABS control module, preventing the pump motor from receiving drive current. 2) "Unable to stop running" — Indicates stuck motor drive relay contacts, failed control chip logic, or an abnormal feedback signal, causing the pump motor to run continuously when not required.

This fault directly disables ABS, ESP, and TCS functions.

In extreme cases, it affects brake pedal feel (hardening or abnormal pulsation).

This is a Level 2 fault affecting driving safety.

The C002004 fault code on the Atto 8 indicates a functional fault in the ABS/ESC Hydraulic Control Unit return pump motor. Severity is medium because ABS, ESP, and TCS may be disabled, and brake pedal feel may change. Common causes include a damaged recirculation pump motor or a failed ABS control module drive circuit, relay, or MOSFET. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Recirculation pump motor damaged: Excessive motor carbon brush wear causing poor contact, a burnt commutator, or an open/short circuit in the armature winding prevents the motor from starting or causes weak operation.
  • 2ABS control module drive circuit fault: The motor drive chip (H-bridge circuit), power MOSFET, or relay contacts inside the control module are burnt out or stuck, causing a failure to drive the motor or disconnect the motor power supply.
  • 3Power supply and wiring issues: Blown ABS hydraulic unit high-current power supply fuse (usually 30A-60A), loose or oxidized power wiring connections, or poor ground wiring contact, causing insufficient motor supply voltage or abnormal control signals.
  • 4Mechanical seizure: Overdue brake fluid replacement causes the internal valve body or pump mechanism to seize. This generates excessive stall current at motor startup, triggering overcurrent protection or burning out the drive circuit.
  • 5Control module connector fault: Water ingress, oxidation, terminal back-out, or poor contact at the multi-pin connector between the ABS control module and the hydraulic unit causes interrupted or abnormal motor drive signal transmission.
What To Do
  • 1
    Fault confirmation and freeze frame recording: Use a dedicated BYD diagnostic tool (VDS2000/VDS6000 or Launch X431) to read fault code C002004. Record freeze frame data (vehicle speed, voltage, temperature, etc.) and confirm whether the fault frequency is intermittent or continuous.
  • 2
    Basic circuit check: Check if the ABS pump high-current fuse in the engine compartment fuse box (marked ABS MOTOR or similar, usually 40A-60A) is blown. Measure the power supply (BAT+), ignition power (IG+), and ground (GND) voltages at the ABS control module connector. Verify the battery voltage is normal (above 12.6V).
  • 3
    Actuator test: Use the diagnostic tool to perform the 'Return Pump Active Test' and listen for the motor running inside the hydraulic unit. If there is no sound, measure the motor resistance. If the motor runs continuously and fails to stop, diagnose a stuck drive circuit.
  • 4
    Physical connection check: Disconnect the ABS control module connector. Inspect the terminals for green oxidation, backed-out pins, enlarged pin holes, or signs of water ingress. Clean the connector with electronic contact cleaner, apply conductive grease, and reconnect it. Clear the fault code and road test the vehicle to confirm if poor contact caused the intermittent fault.
  • 5
    Motor resistance measurement (specialized procedure): If certified for ABS repair, disassemble the hydraulic control unit and use a multimeter to measure the resistance between the two return pump motor terminals. Normal value: 2-5 Ω (varies slightly by model). Infinite resistance (open circuit) or resistance near 0 Ω (short circuit) confirms a faulty motor.
  • 6
    Assembly replacement and matching: If testing confirms an internal hydraulic unit fault, replace the ABS hydraulic control unit assembly (Note: BYD models usually require replacing the complete assembly; the part number must match the VIN). After installation, use the diagnostic tool to perform: ① Brake system bleeding procedure (sequence: right rear - left rear - right front - left front); ② ESC system calibration/configuration; ③ Wheel speed sensor signal check.
  • 7
    Functional verification: Clear all fault codes and perform a road test: ① At low speed (below 30 km/h), brake hard and confirm normal ABS operation with brake pedal pulsation; ② At medium to high speed (above 60 km/h), drive in an S-pattern and confirm normal ESP intervention; ③ Verify the instrument cluster ABS/ESP warning lights are off and confirm fault code C002004 does not return.
Real-World Cases
BYD DTC AI Analysis

2014 BYD Qin Hybrid: Oxidized ABS connector causing DTC C002004

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Vehicle: 2014 BYD Qin (petrol-electric hybrid). Symptoms: ABS and ESP warning lights stayed on. Scanner read fault code C002004 (return pump error). Repair: Checked ABS fuse — normal. Ran return pump actuator test; no response. Pulled ABS control module connector and found green oxide inside (likely from washing or wading). Several pins were corroded. Cleaned connector pins with electronic contact cleaner, applied conductive grease, and reconnected. Cleared codes and test drove — fault cleared temporarily. Two days later, fault returned. Inspection showed the return pump motor inside the hydraulic unit had overheated and failed from the prolonged poor contact. Final repair: Replaced ABS hydraulic control unit assembly. Performed brake bleed and ESC calibration after installation. Fault completely resolved.
Original source ↗
BYD DTC AI Analysis

BYD Song DM ABS return pump motor burned out, assembly replaced.

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Vehicle: 2017 BYD Song DM (plug-in hybrid). Symptoms: ABS, ESP and TCS warning lights illuminated suddenly while driving. Fault code: C002004. No ABS kickback felt during emergency braking; brake pedal stiff. Repair: Scan tool confirmed C002004; code would not clear. ABS pump supply voltage: 12.4 V (normal). Connector condition good. Return pump active test: relay clicked but motor did not run. Diagnosed as motor body or drive circuit fault. The BYD Song DM uses an integrated ABS/ESC assembly (Bosch or Continental system); the return pump motor and ECU are inside the hydraulic unit and cannot be replaced separately. Replaced ABS hydraulic control unit assembly (PN 5A-XXXXXX). Used Launch X431 to bleed brake system and configure ESC. Road test: ABS function normal.
Original source ↗
BYD DTC AI Analysis

BYD F3: Return pump relay stuck, causing continuous pump operation

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Vehicle: 2015 BYD F3 (Bosch 8.1 ABS). Symptoms: ABS pump runs continuously with a buzzing noise after the ignition is switched on, keeps running briefly after the ignition is switched off, and the ABS warning light stays on. Fault code: C002004 (Unable to Stop Running). Repair: This fault manifests as "unable to stop running" — the opposite of the common "unable to run" failure. Inspection of the ABS control module revealed the return pump relay contacts had fused together, keeping the pump motor powered continuously. The F3 ABS assembly is interchangeable with certain Volkswagen models and readily available on the used market. The owner chose a salvaged ABS assembly (cost: approximately 400 yuan). Replacement cleared the fault — the ABS warning light went out and the pump now operates normally (running only during ABS activation or self-check).
Original source ↗
BYD DTC AI Analysis

Sticking return pump on BYD e5 commercial vehicle caused regenerative braking failure

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Vehicle: 2018 BYD e5 (battery electric taxi, 180,000 km). Symptoms: Dashboard displayed "Check Braking System" and "Check ESP", DTC C002004, regenerative braking completely inoperative, significant range loss. Diagnosis: The e5 uses an Electro-Hydraulic Brake (EHB) system; the ABS return pump modulates hydraulic pressure for regenerative braking. The scan tool read C002004; the pump motor showed no response during the actuator test. Disassembled the ABS hydraulic unit and found the return pump motor carbon brushes completely worn from prolonged high-load operation (frequent taxi braking). The brake fluid had not been changed for over 50,000 km, causing poor lubrication and mechanical seizure inside the pump. Fix: Replaced the ABS hydraulic control unit assembly, replaced the brake fluid (DOT4), and calibrated the ESC system. Result: Regenerative braking restored, fault codes cleared.
Original source ↗
BYD DTC AI Analysis

BYD Qin Pro EV ABS Control Module Circuit Board Repair Case

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Vehicle: 2019 BYD Qin Pro EV (battery electric). Symptoms: ABS and ESP warning lights on. Scan tool showed DTC C002004 (return pump error). ABS pump self-test noise at startup, but ABS inoperative while driving. Repair: Checked ABS power and ground — normal. No connector oxidation. Oscilloscope check revealed no PWM drive signal from control module to return pump. Internal motor driver chip (H-bridge) faulty. Dealership quoted ~¥3,200 for complete ABS assembly replacement; owner chose professional electronics repair. Disassembled ABS control module (ECU separated from hydraulic unit), replaced internal motor driver chip (MOSFET), resealed and reinstalled. Cleared DTCs and performed brake bleed. Road test confirmed fault resolved; ABS function restored.
Original source ↗
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.