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 — Seal U

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 — Seal U
  • 112V low-voltage battery aging, low charge, or performance degradation (SOC below 30% or SOH below 80%, static voltage below 12.4V)
  • 2Poor connection, oxidation, or corrosion in the ABS hydraulic modulator assembly power supply circuit, or a blown ABS fuse (usually 30A/40A/60A) or poor fuse contact in the front compartment power distribution 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) causing the 12V system voltage to drop below 13.5V or fluctuate excessively.
  • 5Internal power management module fault in the ABS hydraulic modulator with ECU (e.g., damaged voltage regulator chip, failed filter capacitor, or internal circuit short)
What To Do
  • 1
    Basic power supply check: Measure 12V battery static voltage (≥12.4V) and dynamic voltage (≥13.5V after engine start or in READY mode). Check battery terminals and ABS fuses (FB-02 or ABS1/ABS2) for oxidation and verify tightening torque (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, use the sectional method to check the power supply wiring harness for a voltage drop (battery positive terminal → front compartment fuse box → ABS module connector).
  • 3
    Ground circuit check: Measure the resistance between the ABS module ground pins (e.g., B04-13/38) and the main body ground point. Resistance must be less than 0.1Ω. Check and clean the dedicated ABS ground points G101/G102 on the left front side member or firewall. Verify the connections are free of oxidation, paint, or looseness.
  • 4
    Dynamic load test: Start the vehicle and turn on high-power loads such as headlights, air conditioning, and defroster. Use the diagnostic tool to read 'System Voltage' in the ABS data stream. Verify the voltage remains 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 operations at various vehicle speeds. If C1102 reappears, replace the ABS hydraulic modulator assembly with ECU. After replacement, use VDS to perform yaw rate sensor calibration, G-sensor calibration, and brake line bleeding.
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 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.