AU
C1101

DTC C1101 indicates the ABS (Anti-lock Braking System) ECU detected its 12V low-voltage supply exceeds the system safety threshold (typically >16V-18V, depending on vehicle calibration) — Atto 8

Braking System

DTC C1101 indicates the ABS (Anti-lock Braking System) ECU detected its 12V low-voltage supply exceeds the system safety threshold (typically >16V-18V, depending on vehicle calibration).

In BYD new energy vehicles, the DC-DC converter converts high-voltage traction battery power to supply 12V to the ABS.

When the ECU detects an abnormally high supply voltage, it triggers the overvoltage protection mechanism.

This may cause ABS, ESP, EBD, HHC (Hill Hold Control), and Automatic Emergency Braking functions to fail or enter a degraded mode.

Unlike a high-voltage insulation fault, this indicates a power management abnormality in the 12V low-voltage system, which directly affects braking safety performance.

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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1DC-DC converter fault: The internal voltage regulation circuit of the DC-DC module, which converts high voltage to 12V, fails. The output voltage exceeds 15V-16V, causing ABS ECU power supply overvoltage.
  • 212V battery aged or damaged: Increased internal resistance or battery plate sulfation causes terminal voltage to surge during charging, resulting in abnormally high system voltage.
  • 3ABS module internal power supply monitoring circuit fault: ECU internal voltage sampling circuit or A/D converter fault generates an incorrect overvoltage signal (false alarm).
  • 4Abnormal charging system voltage regulation: A faulty voltage regulator in the DC-DC converter or generator (hybrid models) causes an uncontrolled rise in output voltage.
  • 5Poor contact in power supply or ground circuit: High resistance in the ABS module power (+B) or ground (GND) wire causes an abnormal voltage increase at the ECU sampling point.
What To Do
  • 1
    Use a diagnostic tool to read fault codes and record freeze frame data. Confirm the specific voltage, vehicle speed, and operating conditions when the fault occurred. Check for accompanying power-related fault codes (such as Type U communication faults or Type B body faults).
  • 2
    Measure the 12V battery status: static voltage should be 12.4-12.8V. After starting the vehicle or powering on, measure the DC-DC output terminal voltage. Normal voltage is 13.5-14.5V. If the voltage remains >15V, diagnose a DC-DC fault.
  • 3
    Check the DC-DC converter: Inspect the high-voltage connection from the high-voltage distribution box to the DC-DC converter and measure the voltage at the DC-DC low-voltage output terminal. If the output voltage is abnormal but the input voltage is normal, replace the DC-DC converter.
  • 4
    Check the ABS module power supply circuit: Disconnect the Langjixie ABS module connector. Measure the voltage between Pin 1 (+B constant power) and body ground; voltage must be within the normal range. Measure the resistance between Pin 13/25 (ground) and the body; resistance must be <1Ω. Check the voltage drop across the power supply and ground circuits; voltage drop must be <0.5V.
  • 5
    Check the wiring harness and connectors: Focus on the wiring harness waterproofing near the ABS pump in the engine compartment. Inspect for water ingress, oxidation, spread terminals, or poor contact, especially on vehicles exposed to deep water or heavy rain.
  • 6
    ABS module self-check: After confirming normal supply voltage (13.5-14.5V), if C1101 fails to clear or frequently returns, the ABS assembly internal power supply monitoring circuit has failed. Replace the ABS hydraulic modulator assembly with ECU.
  • 7
    Repair verification: After replacing the relevant components, clear the fault code and perform at least 5 ignition cycle self-checks. Perform a road test (including braking at speeds above 40 km/h) to confirm the ABS functions normally and the fault code does not return.
Real-World Cases
BYD DTC AI Analysis

BYD E2 DC-DC converter fault causing ABS overvoltage warning

✓
A 2020 BYD E2 with 30,000 km had the ABS, ESP and EBD warning lights illuminate simultaneously. The scan tool showed DTC C1101 System High Voltage. Measuring the 12V battery revealed 16.2V in READY mode, fluctuating with accelerator pedal position. The DC-DC converter output was unstable. Replaced the DC-DC module inside the high voltage distribution box; voltage returned to 14.1V. Cleared the fault codes; fault resolved.
BYD DTC AI Analysis

BYD Yuan EV auxiliary battery ageing caused C1101 fault

✓
2019 Yuan EV. Brake system warning light illuminated intermittently in winter. Diagnostic scanner retrieved historical fault code C1101. Testing revealed the 12V battery static voltage was 12.1V (low), spiking to 15.8V momentarily during charging. Replaced the 12V battery; charging voltage stabilised at 14.2V and the fault has not recurred. Plate sulfation increased internal resistance and reduced charge acceptance, causing momentary system overvoltage that triggered protection.
BYD DTC AI Analysis

ABS hydraulic modulator internal circuit fault — false positive

✓
BYD E1 accident repair. C1101 fault code appeared after replacing the front compartment wiring harness. Measured power supply voltage at 13.8V (normal). Found water ingress and oxidized pins at the ABS pump connector. Cleaned the pins but the fault persisted. Disassembled the ABS unit and found the power management chip on the ECU board damaged. Replaced the ABS hydraulic modulator assembly with integrated ECU; fault cleared. Suspect internal circuit damage from reverse polarity connection during the accident or repair.
BYD DTC AI Analysis

Loose ground point caused abnormal voltage detection

✓
The BYD E3 ABS warning light occasionally lit up on rough roads; the scanner read C1101. Inspection found the battery negative ground cable loose at the front compartment ground point G106, with oxidation. After tightening and cleaning the ground point, resistance in the ABS module ground circuit dropped from 3.2Ω to 0.3Ω. A 20 km road test showed no fault recurrence. A high-resistance connection caused ECU reference ground potential drift, leading to incorrect supply voltage calculations.
Other Atto 8 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.