AU
C108C08

DTC C108C08 indicates the ABS control unit detects a logic error or implausibility fault in the reverse switch signal (Signal Invalid) — Seal 6 EV

Braking System

DTC C108C08 indicates the ABS control unit detects a logic error or implausibility fault in the reverse switch signal (Signal Invalid).

In the BYD E5, the HHC (Hill Hold Control) system monitors the reverse signal in real time to determine vehicle travel direction.

When the vehicle is in R gear, the reverse switch closes (signal = '1'), and the HHC system activates the hill reverse assist logic.

If the ABS module detects a '1' reverse signal while the vehicle moves forward (vehicle speed > 0), or a '0' signal while reversing in R gear, it registers a signal fault.

This fault disables the HHC function in reverse mode.

The vehicle risks rolling back when reversing on a slope because the system releases braking force prematurely.

Simultaneously, the system illuminates the ABS/ESC fault warning lamp, disables specific body stability functions, and enters a safety fallback mode.

The C108C08 fault code on the Seal 6 EV indicates the ABS control unit has detected an invalid or implausible reverse switch signal. This can affect Hill Hold Control in reverse and may trigger ABS/ESC warning lights. Severity is medium, as the vehicle could roll on a slope when reversing. Common causes include a faulty reverse switch or poor connector contact from water ingress or loose pins. 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 6 EV
  • 1Reverse switch internal contacts burned, oxidized, or mechanically stuck, preventing the switch from opening or closing normally.
  • 2Water ingress, loose connection, or backed-out pins at the reverse switch connector causing excessive contact resistance or an intermittent open circuit in the signal line.
  • 3Reverse signal circuit (usually from BCM or shift controller to ABS module) shorted to ground, shorted to power, or wiring harness chafed or broken.
  • 4BCM (Body Control Module) software version defect or CAN communication fault causing packet loss or delay during reverse gear signal transmission to the ABS module.
  • 5Internal fault in the ABS control unit signal acquisition circuit (damaged sampling resistor or failed optocoupler), causing incorrect signal readings.
What To Do
  • 1
    Connect the VDS diagnostic tool to access the ABS system and read the 'Reverse switch status' and 'Current gear' parameters in the data stream. Compare these with the actual gear selector position to confirm signal synchronization. Simultaneously check for accompanying U-class communication fault codes.
  • 2
    Visually inspect the reverse gear switch mounting (located near the motor controller or on the gearshift mechanism assembly) and connector sealing. Check specifically for signs of water ingress or green oxidation on the terminals. If necessary, clean and spray with WD-40 electrical contact cleaner.
  • 3
    Disconnect the reverse switch connector and use a multimeter to measure switch body continuity. With N or D engaged, the circuit must be open (resistance >10MΩ). With R engaged, the circuit must be closed (resistance <1Ω). If abnormal, replace the reverse switch assembly.
  • 4
    Measure the reverse switch signal wire voltage. With the ignition ON and the switch disconnected, the signal wire should have a 12V reference voltage (the BCM or ABS supplies pull-up power). When engaging reverse gear and the switch closes, the voltage should drop to <0.5V (ground). If the voltage is abnormal, check the circuit for a short to ground, short to power, or open circuit.
  • 5
    Check the CAN communication circuit between the BCM and ABS module: measure the resistance between OBD terminal 6 (CAN-H) and terminal 14 (CAN-L). The resistance must be 60Ω±5Ω, and the voltage to ground must be 2.5V-3.5V and 1.5V-2.5V respectively. If the resistance is abnormal, check the terminating resistor and inspect the circuit for short circuits.
  • 6
    If the wiring and switch are normal, check the BCM software version (early versions have a reverse gear signal transmission delay bug) and update to the latest version; or reset the ABS control unit coding and calibrate the HHC function.
  • 7
    After repairing or replacing the faulty component, clear the fault code and perform an R-gear hill-start test on a slope (>3°). Confirm the HHC function operates normally (maintains braking force for 2-3 seconds after releasing the brake pedal) and the fault code does not recur.
Real-World Cases
BYD DTC AI Analysis

Reverse switch contacts burnt on 2018 E5, causing intermittent signal loss

✓
Dashboard ABS and ESC warning lights lit up intermittently with no driving abnormalities. Scanned DTC C108C08. Live data showed the reverse gear signal occasionally jumped to OFF when selecting reverse. Removed and inspected the reverse switch located above the motor controller; found the internal silver contacts severely burnt and blackened, with contact resistance fluctuating between 0.5-50Ω. Replaced with a genuine BYD reverse switch (part number: 2320910-00). Signal now stable, warning lights went out, and Hill Hold Control (HHC) function tested OK when reversing on a slope.
Original source ↗
BYD DTC AI Analysis

2019 E5 boot wiring harness chafed at hinge, causing signal circuit open

✓
The owner reported judder when reversing and the ESC warning light staying on constantly. The scanner read DTC C108C08. Static testing of the reverse switch showed normal operation, but shaking the vehicle reproduced the fault. Inspection found that on the E5 model, the reverse signal harness runs through the left wing into the boot, where the insulation had chafed through at the bend near the left boot hinge. The copper core had fractured, leaving only two strands connected, causing the signal to drop out when the vehicle vibrated. Stripped back the harness, re-soldered the wires, added corrugated tubing and secured the routing. Fault completely eliminated.
Original source ↗
BYD DTC AI Analysis

BCM software bug caused conflict between reverse gear signal and vehicle speed logic.

✓
While driving normally, the vehicle suddenly set DTC C108C08 and HHC failed. Checked the reverse switch and wiring continuity—both normal. Measured reverse signal voltage at the BCM output—normal. However, the ABS module data stream showed signal logic inconsistent with vehicle speed. Consulted the BYD TSB: BCM software V1.1 on this batch has a CAN signal transmission timing error. Upgraded the BCM to V2.0. Signal synchronisation returned to normal; cleared the fault code—no recurrence.
Original source ↗
Other Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.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 Seal 6 EV →Seal 6 EV 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.