AU
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.

The C004008 fault code on the Seal U 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, Auto Hold, ESP functions, and in some cases Ready mode. Common causes include internal switch wear or failure, and water-damaged 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 — Seal U
  • 1Burnt internal contacts, spring fatigue, or Hall element failure in the brake pedal switch body, preventing normal on/off signal switching.
  • 2Aged switch connector seal allows water ingress, causing oxidation (green copper corrosion) or terminal pin back-out, resulting in intermittent poor contact.
  • 3Wiring harness insulation chafed or damaged near the firewall, steering column, or pedal mechanism, causing a short to ground or short to power.
  • 4Worn pedal mechanism rubber pad or deformed mounting bracket causes an excessive gap between the switch push rod and the pedal (normal preload gap: 0.5-1.0 mm), resulting in signal trigger threshold failure.
  • 5Internal fault in the ABS/ESC control unit signal acquisition circuit or software logic error (less common; consider only after ruling out external wiring).
What To Do
  • 1
    Use the diagnostic tool to read the freeze frame data. Verify the vehicle speed, pedal status, and primary and secondary switch signal logic at the time of the fault to determine whether it is a static or dynamic intermittent fault.
  • 2
    Visually inspect the physical installation of the brake pedal switch. Confirm the retaining clip is intact and the push rod shows no excessive wear. Check the wiring harness for interference during full pedal travel and at 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/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 must open and the auxiliary contact must close. When pressing the pedal, the states must 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). Focus inspection on the wiring harness at the firewall grommet and under the dashboard.
  • 6
    After replacing the 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 erratic changes.
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
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.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.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.