AU
C004008

DTC C004008 indicates the ABS/ESC control unit detected an abnormal Brake Pedal Switch signal — Qin Plus

Braking System

DTC C004008 indicates the ABS/ESC control unit detected an abnormal Brake Pedal Switch signal.

The Brake Pedal Switch typically contains primary and secondary contact sets (a normally open/normally closed combination) to provide hard-wired driver braking intent signals to the Vehicle Control Unit (VCU) and the ABS module.

These signals directly affect brake light control, regenerative braking cancellation, Auto Hold logic, and vehicle power-up (Ready) permission.

This DTC sets when the ABS module detects the switch signal voltage, logic combination (whether primary and secondary signals are mutually exclusive), or signal change rate exceeds calibrated thresholds.

Potential causes include a single-circuit open, simultaneous dual-circuit short, signal desynchronization, or internal switch failure.

A persistent fault causes abnormal regenerative braking, ESP function degradation, and brake lights stuck on or off.

Extreme conditions trigger vehicle limp mode or prevent vehicle power-up.

The C004008 fault code on the Qin Plus indicates an abnormal brake pedal switch signal detected by the ABS/ESC control unit. Severity is medium, as it can affect brake lights, regenerative braking, ESP, Auto Hold, and vehicle Ready status. Common causes include internal switch wear or failure, and water ingress or corroded connector terminals. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1Burnt internal contacts, spring fatigue, or Hall element failure in the brake pedal switch body prevents normal on/off signal switching.
  • 2Aged switch connector seal allows water ingress and oxidation (green copper corrosion) or terminal pin back-out, causing intermittent poor contact.
  • 3Chafed or damaged wiring harness insulation near the firewall, steering column, or pedal mechanism causes a short to ground or short to power.
  • 4Worn pedal mechanism rubber pad or deformed mounting bracket creates an excessive gap between the switch push rod and the pedal (normal preload gap: 0.5-1.0 mm), causing marginal signal trigger failure.
  • 5ABS/ESC control unit internal signal acquisition circuit fault or software logic error (less common; consider only after ruling out external wiring).
What To Do
  • 1
    Use the diagnostic tool to read freeze frame data. Confirm vehicle speed, pedal status, and main and sub switch signal logic at the time of the fault. Determine whether the fault is static or dynamic intermittent.
  • 2
    Visually inspect the physical installation of the brake pedal switch. Confirm the retaining clip is not broken and the push rod shows no excessive wear. Check for wiring harness interference during full pedal travel and at the steering wheel limit positions.
  • 3
    Disconnect the switch connector and measure the power supply pin voltage (should be 12V/B+) and ground circuit resistance (should be <1Ω) to rule out power or ground faults.
  • 4
    Use a multimeter to measure the continuity logic of the main and auxiliary switch contacts: when releasing the pedal, the main contact should be open and the auxiliary contact closed; when pressing the pedal, the states should reverse. Alternatively, use an oscilloscope to capture signal changes and check for glitches or delays.
  • 5
    Measure continuity between the switch wiring harness and the ABS pump connector (resistance <1Ω) and insulation to ground/power (resistance >10MΩ). Specifically inspect the wiring harness at the firewall grommet and under the dashboard.
  • 6
    After installing the new switch, perform brake pedal position learning or ESC system calibration on applicable models. Clear the fault code and perform a road test to verify the brake signal in the data stream synchronizes with pedal movement and shows no jumps.
Real-World Cases
BYD DTC AI Analysis

BYD E5 - Internal open circuit in brake pedal switch

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Symptoms: Vehicle would not enter Ready mode. Instrument cluster displayed "Check braking system" and brake lights did not illuminate. Current fault code C004008 present. Data stream showed both brake pedal primary and secondary signals as "not depressed". Diagnosis: Disconnected pedal switch connector. Measured switch internal contacts with multimeter. Resistance remained infinite with pedal both depressed and released (normal: 0Ω/∞Ω toggle). Confirmed switch internal contacts burnt open. Solution: Replaced brake pedal switch (part cost: 45-65 yuan). Reconnected connector. Vehicle powered up normally and brake lights functioned. Cleared fault codes; ABS self-check passed.
BYD DTC AI Analysis

BYD Qin Pro DM - Water ingress into connector caused oxidation and intermittent fault

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Symptoms: ABS/ESP warning light intermittently illuminates while driving, accompanied by sudden loss of regenerative braking. DTC C004008 stored as history. Occurs frequently in rainy weather or after water crossings. Diagnosis: Inspected the switch connector beneath the dashboard on the pedal bracket. Found green corrosion on internal pins from car wash or AC water leakage. Wiggling the harness caused brake signal fluctuation in live data. Solution: Cleaned connector pins with electrical contact cleaner, applied conductive grease, and replaced the seal. Reconnected the plug and secured the harness above the bracket with cable ties to prevent sagging. No recurrence at one-month follow-up.
BYD DTC AI Analysis

BYD Song PLUS EV - Pedal mechanism wear causing abnormal signal gap

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Symptoms: Above 80 km/h or during light braking, the instrument cluster intermittently displayed "Check ESP System". DTC C004008 (signal implausible) set. City driving at low and medium speeds remained normal. Diagnosis: Inspected the brake pedal mechanism and found the pedal return buffer rubber pad severely worn. The switch plunger could not fully extend after pedal release, leaving the primary and secondary signals marginal. Measured switch plunger travel at only 0.2 mm (specification requires >0.5 mm). Solution: Replaced the pedal mechanism buffer pad. Readjusted the switch mounting position to ensure 0.8 mm preload clearance. Tightened the locknut to 8–10 N·m. Road-tested under high-speed braking and light braking conditions; fault resolved.
BYD DTC AI Analysis

BYD Tang DM - Wiring harness interference with steering column causes wear and short circuit

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Symptoms: Intermittent failure to enter Ready mode. Instrument cluster simultaneously displays "Check Brake System" and "Check ESP." Scanner reads DTCs C004008 (main signal) and C004108 (secondary signal). Diagnosis: Inspected firewall-to-dashboard harness. Retaining clip had detached, allowing harness to rub against steering column universal joint when turning wheel or pressing pedal. Cutting open the sheath exposed copper strands on two signal wires with short-circuit damage. Solution: Cut out damaged 20cm section. Resoldered wires and applied double-layer heat shrink tubing. Rerouted harness and secured with dedicated clip to bracket away from steering column. Verified no interference through full steering and pedal travel.
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.