AU
C108C08

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

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 Qin Plus indicates a medium-severity ABS/ESC fault where the reverse switch signal is invalid or inconsistent with vehicle movement. This can disable reverse hill hold assist and may trigger ABS/ESC warning lights. Common causes include a stuck or oxidized reverse switch, water ingress, loose connectors, or damaged pins. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Burnt, oxidized, or mechanically stuck internal contacts prevent the reverse switch from opening or closing properly.
  • 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 circuit.
  • 3Reverse signal circuit (usually from BCM or gearshift controller to ABS module) shorted to ground, shorted to power, or wiring harness worn or broken.
  • 4BCM (Body Control Module) software version defect or CAN communication fault causes packet loss or delay when transmitting the reverse gear signal 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, access the ABS system, and read the 'Reverse gear 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 reverse switch installation (located near the motor controller or on the gearshift mechanism assembly) and connector sealing. Specifically check for signs of water ingress or green oxidation on the terminals. If necessary, clean the terminals and spray WD-40 electronic contact cleaner.
  • 3
    Disconnect the reverse switch connector and use a multimeter to measure switch body continuity: with N/D engaged, the circuit must be open (resistance >10MΩ); with R engaged, the circuit must have continuity (resistance <1Ω). If abnormal, replace the reverse switch assembly.
  • 4
    Measure the reverse switch signal wire voltage: with ignition ON and the switch disconnected, the signal wire should show a 12V reference voltage (BCM or ABS provides pull-up power); when shifting into R gear, the switch closes and 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). Resistance must be 60Ω±5Ω, and voltage to ground must be 2.5V-3.5V and 1.5V-2.5V respectively. If resistance is abnormal, check the terminating resistor and circuit for a short.
  • 6
    If the wiring and switch are normal, check the BCM software version (early versions contain a bug causing delayed reverse gear signal transmission). 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 reappear.
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 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.