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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 — Qin Plus

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 Qin Plus indicates an abnormal communication signal between the electronic parking brake (EPB) module and the ABS/ESC module. It is a medium-severity issue that can affect AUTO HOLD, hill hold, or parking brake operation. Common causes include an internal EPB module fault or unstable power/ground supply. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1EPB control module internal circuit fault: MCU processor damage, power management chip failure, 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), loose fuse connection, oxidized relay contacts, or excessive ground circuit contact resistance (>1Ω)
  • 3CAN bus communication fault: Short or open circuit in Power CAN-H and CAN-L lines, terminal 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 causes signal voltage drift, or the switch wiring harness wears and breaks near the steering column.
  • 5Wiring harness connector issue: Oxidation, terminal back-out, or poor contact at the EPB module connector pins beneath 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 from when the fault occurred, such as 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 9.3-16.9V. Measure the ground resistance; it must be less than 1Ω. Check the fuse and relay status.
  • 3
    Check CAN bus communication: Measure voltage at diagnostic connector terminals 6 (CAN-H) and 14 (CAN-L) (CAN-H: 2.5-3.5V, CAN-L: 1.5-2.5V). Disconnect the battery and measure terminal 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 wiring harness continuity between the switch and the module. Specifically inspect the wiring harness beneath 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 the 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). 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 brake 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

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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 Qin Plus 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.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.B173BDTC 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.
Common problems reported for Qin Plus →Qin Plus 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.