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 — Seal U

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.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1EPB control module internal circuit fault: damaged MCU processor, failed power management chip, or corrupted 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 contact resistance in the ground circuit (>1Ω)
  • 3CAN bus communication fault: Powertrain CAN-H and CAN-L lines shorted or open, terminating resistor drift (deviation 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 wear or breakage near the steering column.
  • 5Wiring harness connector issue: Oxidation, backed-out pins, or poor contact at the EPB module connector pins under the center console, near the seat track, or at the firewall wiring grommet.
What To Do
  • 1
    Use a dedicated BYD diagnostic tool (VDS or ED400) to read the complete DTC information and freeze frame data. Record key parameters at the time of the fault, such as vehicle speed, system voltage, ambient temperature, and gear position.
  • 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 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 termination resistance (standard: approx. 60Ω); use an oscilloscope to check for waveform distortion.
  • 4
    Check the EPB switch signal: Measure switch resistance and output voltage in different positions (release/apply). Check wiring harness continuity between the switch and the module. 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 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 armrest 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 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

✓
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

✓
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

✓
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 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.