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C004008

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

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.

4
Cases Logged
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 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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.