AU
C1102

DTC C1102 indicates the ABS (Anti-lock Braking System) electronic control unit (ECU) detects an operating voltage below the minimum system-calibrated threshold (typically 9V-10 — Atto 3

Braking System

DTC C1102 indicates the ABS (Anti-lock Braking System) electronic control unit (ECU) detects an operating voltage below the minimum system-calibrated threshold (typically 9V-10.5V, depending on vehicle calibration).

This fault indicates an abnormal power supply to the ABS hydraulic modulator assembly, potentially causing active safety systems including ABS, ESP (Electronic Stability Program), EBD (Electronic Brakeforce Distribution), and Automatic Emergency Braking (AEB) to enter a degraded mode or fail completely.

In BYD new energy vehicles, this fault typically results from a discharged 12V low-voltage battery, excessive voltage drop in the power supply circuit, high ground circuit resistance, or an internal ABS ECU power management module failure.

This critical fault compromises driving safety and requires immediate repair.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 112V low-voltage battery aging, low charge, or performance degradation (SOC below 30% or SOH below 80%, static voltage below 12.4V)
  • 2Loose connection, oxidation, or corrosion in the ABS hydraulic modulator assembly power supply circuit, or a blown or poorly connected ABS fuse (usually 30A/40A/60A) in the front compartment fuse box.
  • 3Loose, oxidized, or paint-covered ABS ECU ground points causing increased ground resistance (common ground points: G101 left front side member, G102 firewall)
  • 4Abnormal output voltage from the DC-DC converter (NEV) or alternator (ICE vehicle) causes the 12V system voltage to drop below 13.5V or fluctuate excessively.
  • 5Internal power management module fault in the ABS hydraulic modulator with ECU (such as a damaged voltage regulator chip, failed filter capacitor, or internal short circuit)
What To Do
  • 1
    Basic power supply check: Measure the 12V battery static voltage (≥12.4V) and dynamic voltage (≥13.5V after engine start or in READY mode). Inspect the battery terminals and ABS fuses (FB-02 or ABS1/ABS2) for oxidation and verify the tightening torque is 8-10 N·m.
  • 2
    ABS module power supply check: Turn the ignition switch to ON. Use a multimeter to measure the voltage between the ABS module power supply pin (e.g., B04-1/25) and body ground. If the voltage is below 10V, check the power supply wiring harness voltage drop segment by segment (from the battery positive terminal → front compartment distribution box → ABS module connector).
  • 3
    Ground circuit check: Measure the resistance between the ABS module ground pins (e.g., B04-13/38) and the body main ground point. The resistance must be less than 0.1Ω. Check and clean the ABS dedicated ground points G101/G102 on the left front longitudinal beam or firewall. Ensure no oxidation, paint, or looseness.
  • 4
    Dynamic load test: Start the vehicle. Turn on high-power loads such as headlights, A/C, and defroster. Use the diagnostic tool to read 'System Voltage' in the ABS data stream. Verify the voltage is within the normal 9V-16V range. If the voltage drops significantly under load, check the DC-DC output stability.
  • 5
    Module self-check and replacement: Clear the fault code and perform a road test (including braking at different vehicle speeds). If C1102 reappears, replace the ABS hydraulic modulator assembly with ECU. After replacement, use VDS to calibrate the yaw rate sensor, calibrate the G-sensor, and bleed the brake lines.
Real-World Cases
BYD DTC AI Analysis

BYD Song - Brake Pedal Switch Signal Fault (C004008)

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Symptoms: Dashboard intermittently displayed "Check ESP System" while driving. Fault occurred randomly and sometimes cleared itself after driving. Diagnosis: Scanned with VDS and retrieved DTC C004008 (brake pedal switch signal fault) from the ESP system. Checked brake pedal switch live data; signal changed abnormally when depressing the pedal, with lag and interruption. Inspected switch mounting position and found slight deformation of the mounting bracket. Measured switch wiring; power, ground and signal continuity were normal, ruling out wiring faults. Solution: Replaced brake pedal switch assembly and adjusted mounting position to set pedal free travel to spec (3-5mm). Cleared codes and road-tested; fault did not recur.
Original source ↗
BYD DTC AI Analysis

BYD Song MAX – Intermittent shift failure (brake signal related)

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Symptoms: Intermittent inability to shift out of Park when pressing the brake pedal. The fault tends to appear after the vehicle sits in Park for 10+ minutes following startup. No warning lights on the instrument cluster. Diagnosis: When the fault occurred, pressing the brake pedal produced no unlocking sound from the shift interlock solenoid. Scanned with VDS: no DTCs stored. However, the left domain controller data stream showed the brake switch signal occasionally did not change when the pedal was pressed. Inspection found a loose connection at the brake pedal switch connector with slight oxidation on the internal pins. Measured supply voltage at 12V (normal), but signal output fluctuated between 3-8V (should toggle 0V/12V). Repair: Cleaned the brake pedal switch connector pins, applied conductive grease, and reconnected. The fault disappeared temporarily. Advised replacing the brake pedal switch assembly for a permanent fix.
Original source ↗
BYD DTC AI Analysis

BYD Song TID - Intermittent Non-Start (Brake Signal Fault)

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Symptoms: Intermittent no-start. Remote key operates locks normally; vehicle powers on to ON mode, but the starter does not engage when pressing the brake pedal and start button. Cluster shows "Check EPB System" and "Check ESP System" warnings; P indicator flashes. Diagnosis: When the fault occurs, the cluster displays multiple system errors, but VDS scans intermittently show no codes or communication failures. Focused inspection of the brake signal system reveals the brake pedal switch outputs dual signals (normally open/normally closed). The two signals occasionally show logic inconsistency—theoretically opposite states, but both reading high simultaneously. Wiring inspection finds chafing on the harness between the switch and front compartment fuse box at the firewall; exposed copper wires cause signal crosstalk. Testing the switch shows burnt internal contacts. Resolution: Replaced the brake pedal switch assembly. Repaired the damaged wiring, rewrapped the harness, and secured routing to avoid rubbing against body metal. Cleared all fault codes and performed brake signal calibration. Test drove for one week with no recurrence.
Original source ↗
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.