AU
C108C08

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

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.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Burnt, oxidized, or mechanically stuck internal contacts in the reverse switch prevent the switch from opening or closing properly.
  • 2Water ingress, loose connection, or backed-out pins in the reverse switch connector, causing excessive contact resistance or an intermittent open circuit in the signal circuit.
  • 3Reverse signal circuit (usually from BCM or gear shift controller to ABS module) shorted to ground, shorted to power, or wiring harness worn or broken.
  • 4A BCM (Body Control Module) software version defect or CAN communication fault causes packet loss or delay during reverse gear signal transmission to the ABS module.
  • 5Internal signal acquisition circuit fault in the ABS control unit (damaged sampling resistor or failed optocoupler), causing incorrect signal readings.
What To Do
  • 1
    Connect the VDS diagnostic tool, access the ABS system, and read the 'Reverse switch status' and 'Current gear' data stream parameters. 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 mounting condition of the reverse gear switch (located near the motor controller or on the gearshift mechanism assembly) and the connector seals. Inspect closely for signs of water ingress and green oxidation on the terminals. Clean and spray WD-40 electrical contact cleaner if necessary.
  • 3
    Disconnect the reverse switch connector and use a multimeter to measure switch continuity. Shift into N or D gear; the circuit must be open (resistance >10 MΩ). Shift into R gear; the circuit must be closed (resistance <1 Ω). 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 provides the pull-up power). When shifting into R 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 wiring between the BCM and ABS module: measure the resistance between OBD connector pin 6 (CAN-H) and pin 14 (CAN-L). The resistance must be 60Ω±5Ω. Measure the voltage to ground; it must be 2.5V-3.5V for CAN-H and 1.5V-2.5V for CAN-L. If the resistance is abnormal, check the terminating resistor and wiring 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). Update to the latest version, or perform an ABS control unit coding reset and HHC function calibration.
  • 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 (the system maintains braking force for 2-3 seconds after releasing the brake pedal) and the fault code does not return.
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 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.