AU
C002004

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

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.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Recirculation pump motor damaged: Excessive motor carbon brush wear causing poor contact, commutator ablation, 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 driver chip (H-bridge circuit), power MOSFET, or relay contacts inside the control module are burnt out or stuck, resulting in failure to drive the motor or cut off 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 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. Excessive stall current at motor startup triggers overcurrent protection or burns 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 interrupts or disrupts 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 DTC C002004. Record freeze frame data (vehicle speed, voltage, temperature, etc.) and confirm if the fault 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. Check 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, proceed to motor resistance measurement; if the motor runs continuously and fails to stop, diagnose a stuck drive circuit.
  • 4
    Physical connection check: Disconnect the ABS control module connector and check the pins for green oxidation, backed-out pins, enlarged pin holes, or signs of water ingress. Clean the connector with electrical 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 (requires professional operation): If qualified for ABS repairs, disassemble the hydraulic control unit and use a multimeter to measure the resistance between the two return pump motor terminals. The normal value is 2-5Ω (varies slightly by model). Infinite resistance (open circuit) or resistance approaching 0Ω (short circuit) confirms the motor itself is damaged.
  • 6
    Assembly replacement and matching: If diagnostics confirm 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 a 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
    Function verification: Clear all fault codes and perform a road test: ① At low speeds (below 30 km/h), apply hard braking to confirm normal ABS operation and pedal pulsation; ② At medium to high speeds (above 60 km/h), perform S-curve driving to confirm normal ESP intervention; ③ Check that 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 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.