AU
C040008

C040008 is a UDS diagnostic protocol chassis system fault code indicating an abnormal communication signal between the EPB (Electronic Parking Brake) control module and the ABS/ESC module — Atto 3

Braking System

C040008 is a UDS diagnostic protocol chassis system fault code indicating an abnormal communication signal between the EPB (Electronic Parking Brake) control module and the ABS/ESC module.

This fault indicates the ABS system received an EPB status signal (e.g., parking motor position, clamping force, switch status, or system ready status) with a validity error, checksum error, signal timeout, or out-of-range value.

This fault may cause AUTO HOLD function failure, abnormal Hill Hold Control (HHC) operation, or failure of the electronic parking brake to apply or release.

Extreme cases compromise driving safety.

The C040008 fault code on the Atto 3 indicates an abnormal communication signal between the electronic parking brake and ABS/ESC module. Severity is medium because it can affect Auto Hold, Hill Hold Control, or parking brake apply/release operation. Common causes include an internal EPB control module fault or unstable power/ground supply. The first recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from 5 recorded cases.
5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1EPB control module internal circuit fault: damaged MCU processor, failed power management chip, or abnormal internal memory data causing incorrect signal output.
  • 2Power supply system fault: unstable EPB module supply voltage (below 9.3V or above 16.9V), poor fuse connection, oxidized relay contacts, or excessive contact resistance in the ground circuit (>1Ω)
  • 3CAN bus communication fault: Short or open circuit in Powertrain CAN-H and CAN-L wiring, terminating resistor drift (deviating from 60Ω), wiring aging, or external electromagnetic interference causing signal distortion.
  • 4EPB switch assembly fault: Internal switch contact oxidation or spring fatigue causing signal voltage drift, or switch wiring harness worn or broken near the steering column.
  • 5Wiring harness connector issue: Pin oxidation, backed-out pins, or poor contact at the EPB module connector under the center console, near the seat rail, or at the firewall wiring grommet.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS or ED400) to read the complete DTC information and freeze frame data. Record key parameters at the time of the fault, including vehicle speed, system voltage, ambient temperature, and gear status.
  • 2
    Check the EPB control module supply voltage: With the ignition switch ON, measure the voltage between the module power terminal and the ground terminal. The standard value is 9.3-16.9 V. Measure the ground resistance; it must be less than 1 Ω. Check the fuse and relay status.
  • 3
    Check CAN bus communication: Measure voltage at diagnostic connector pins 6 (CAN-H) and 14 (CAN-L) (CAN-H: 2.5-3.5V, CAN-L: 1.5-2.5V). Disconnect the battery and measure the terminating resistance (standard approx. 60Ω). Use an oscilloscope to check the waveform for distortion.
  • 4
    Check the EPB switch signal: Measure the switch resistance and output voltage in different positions (release/apply). Check the wiring harness continuity between the switch and the module. Closely inspect the wiring harness under the steering column for wear.
  • 5
    Perform the EPB system function test: operate the parking brake switch through multiple apply/release cycles. Use the diagnostic tool to read motor current (normal <15A), operating time (normal 1.5-2.5 seconds), and clamping force data. Check for mechanical binding.
  • 6
    If all above checks are normal, replace the EPB control module (located under the center console or rear seat) and complete online matching and calibration: use the diagnostic tool to write the VIN and configuration parameters, and perform EPB system initialization learning (some models require braking force calibration).
Real-World Cases
BYD DTC AI Analysis

BYD Qin Hill-Hold Release Failure

✓
2018 BYD Qin DM. EPB worked normally on flat roads, but after parking on >15% slopes and restarting, the cluster displayed "Check Electronic Parking System." DTC C040008 stored. Handbrake failed to auto-release; required a vehicle restart to disengage. Checked EPB module power supply: 12.4V (normal); CAN bus resistance: 59.8Ω (normal); waveform normal. Replaced EPB control module and performed online matching; fault cleared. Root cause: intermittent failure of the internal gradient sensor signal processing circuit prevented the ABS module from receiving valid gradient compensation signal.
Original source ↗
BYD DTC AI Analysis

Oxidised wiring harness connector caused signal interruption in BYD E5

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2019 BYD E5. EPB warning light illuminated intermittently while driving with DTC C040008 stored, accompanied by intermittent ABS warning light activation. Located the EPB module beneath the centre console; pins 6 (CAN-H) and 14 (CAN-L) of the 16-pin connector showed severe green corrosion with contact resistance measuring 3.2Ω (specification <0.5Ω). Cleaned the connector pins, reinforced the socket using a dedicated wiring harness repair tool, applied conductive anti-corrosion paste, and reassembled. Road-tested for 20 kilometres with no recurrence of the fault.
BYD DTC AI Analysis

Loose connection at fuse holder caused unstable EPB supply voltage

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With the vehicle stationary, the dashboard displayed "EPB System Fault". The scan tool retrieved codes C040008 and C040007 (EPB power supply fault). Measured EPB module power terminals; with ignition ON, voltage fluctuated between 7–11V, well below specification. Inspected the EPB 15A fuse in the engine bay fuse box and found loose contact between the fuse blade and base, with fatigue in the internal metal retaining clip. Replaced the engine bay fuse box assembly. Re-measured EPB module supply voltage: restored to 12.6V. Cleared fault codes; system returned to normal.
BYD DTC AI Analysis

Aftermarket device interferes with CAN bus, causing EPB signal error

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BYD Song DM owner reported the electronic parking brake occasionally failed to release after parking, with the dashboard displaying "Check Brake System" and DTC C040008 stored. Inspection revealed a recently installed dash cam below the center console with its power wire mistakenly tapped into the CAN bus. Removed the unauthorized wiring, restored the factory harness layout, re-wrapped the CAN harness with shielding tape, cleared the fault codes, and repeatedly tested the EPB—function normal. Root cause: aftermarket device interference caused data packet loss between the EPB and ABS modules.
BYD DTC AI Analysis

Internal contacts in EPB switch oxidised, causing signal drift

✓
2019 BYD E5. Pressing the EPB switch occasionally produced no response on the instrument cluster. The scanner showed C040008 (EPB signal error) and B1B0008 (switch signal abnormal). Disassembled the EPB switch assembly and found the internal sliding contacts oxidised and blackened. Measured switch resistance in the release position with a multimeter; reading was infinite (spec <5Ω). The switch signal voltage drifted outside the ABS module recognition range. Replaced the EPB switch assembly (part number: K9-3750100). No additional calibration required. Tested all functions—normal.
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.