AU
C012104

DTC C012104 indicates the ABS/ESP electronic control unit (ECU) detected a fault in the solenoid valve relay control circuit — Seal U

Braking System

DTC C012104 indicates the ABS/ESP electronic control unit (ECU) detected a fault in the solenoid valve relay control circuit.

This relay resides inside the hydraulic control unit (HCU) and supplies stable operating power (typically 12V) to the inlet/outlet solenoid valve assembly.

The ECU triggers this fault upon detecting an open or short in the relay coil circuit, or abnormal relay contact feedback voltage (such as output voltage dropping below the threshold after the relay energizes).

This fault disables active safety functions relying on hydraulic regulation, such as ABS, EBD, ESP, and TCS.

The vehicle retains only basic hydraulic braking capability.

In models such as the BYD E5, the HCU integrates this relay.

The relay is not serviceable separately; replace the complete assembly.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Burned internal relay contacts or an open coil circuit in the Hydraulic Control Unit (HCU/ABS pump) prevents the ECU from properly engaging the relay.
  • 2Abnormal power supply, including blown ABS fuse (usually 30A or 40A), low battery voltage (<10V), or poor connection in the power supply circuit.
  • 3Poor ground circuit, particularly loose or oxidized HCU ground terminals (G104/G105) or a broken wiring harness, causing unstable relay operating voltage.
  • 4An internal short circuit or overload in the solenoid valve assembly causes protective relay disconnection or contact sticking, triggering circuit fault monitoring.
  • 5Damaged ECU internal drive circuit, such as relay drive transistor (MOSFET) breakdown or a cold solder joint on the control chip.
What To Do
  • 1
    Safety Preparation and Initial Inspection: Disconnect the high-voltage system (for new energy vehicles). Check if the ABS fuse (30A/40A in the engine compartment fuse box) is blown. Measure the battery voltage (should be >12V). Check the ABS/ESP warning light status on the instrument cluster.
  • 2
    Deep diagnostic scan: Use a VDS or Launch diagnostic tool to read all fault codes and check for accompanying wheel speed sensor faults, such as C0035 or C0040. Perform the 'solenoid valve actuation test', listen for the relay engaging sound, and verify the 'valve relay status' in the data stream is 'ON'.
  • 3
    Wiring and connector check: Measure the voltage between the HCU connector power terminal (constant +B) and ground; it should be 12.4-12.8V. Measure the resistance between the HCU ground harness (usually a black wire ≥4mm²) and body ground; it should be <1Ω. Inspect the connector for water ingress or corrosion.
  • 4
    Relay circuit specific test: Disconnect the HCU connector. Measure the resistance between the relay coil control terminal and power/ground (normal: approx. 50-100Ω). Infinite or 0Ω resistance indicates a faulty internal coil. Measure the relay drive signal waveform with an oscilloscope to verify the ECU sends the pull-in command.
  • 5
    Component replacement and matching: If testing confirms an internal HCU relay fault, replace the complete ABS hydraulic modulator assembly with ECU. After replacement, perform the 'Hydraulic Bleeding' procedure (use the diagnostic tool to drive the solenoid valves for staged bleeding) and the 'G-sensor/Yaw Rate Sensor Calibration'.
  • 6
    Function verification: Clear the fault code, conduct a road test (above 40 km/h), and perform an emergency braking test to confirm normal ABS operation. Check the ESP function (rapid lane-change test to confirm normal intervention). Confirm the warning light is off and the fault code does not recur.
Real-World Cases
BYD DTC AI Analysis

2018 BYD E5: Burnt relay contacts caused ABS failure

✓
The vehicle came in with the instrument cluster showing "Check Brake System" and both ABS and ESP warning lights on. VDS read DTC C012104 (solenoid valve relay circuit fault), which would not clear. We removed the ABS pump (with ECU assembly) and measured the internal relay, finding severely burned contacts causing excessive contact resistance (>5Ω). The ECU detected an abnormal voltage drop and logged the fault. Since the relay is sealed inside the HCU, we replaced the entire ABS hydraulic control unit assembly (part number: EG-3635100). We bled the brake lines and matched the wheel speed sensors, and the fault cleared.
BYD DTC AI Analysis

Poor ground on E5 taxi causes intermittent C012104 fault

✓
An E5 taxi with 120,000 km developed an intermittent ABS warning light that cleared after restarting. The scan tool showed history fault code C012104. Inspection found the HCU ground harness bolt (G104) on the left longitudinal member in the engine compartment loose and oxidized, increasing resistance in the relay circuit. Tightening and polishing the ground terminal, applying conductive paste, and retesting dropped ground resistance from 8Ω to 0.3Ω. One week of tracking showed no fault recurrence.
BYD DTC AI Analysis

Blown fuse caused multiple braking system faults in new energy logistics vehicle

✓
The vehicle failed to power up and the dashboard displayed 'Brake System Fault'. Scanned DTCs C012104 and C003501. Found the ABS main power fuse (40A) blown in the engine bay fuse box. Replaced the fuse and cleared the codes, but it blew again after several kilometres. Found the HCU internal solenoid coil shorted to ground—measured 2Ω between the power terminal and ground. Replaced the ABS assembly and fuse. Fault resolved. Root cause: solenoid ageing caused relay circuit overcurrent.
BYD DTC AI Analysis

BYD Qin Pro DM-i ESP hydraulic unit internal fault

✓
While driving on a wet road, the ESP intervened abnormally and the instrument cluster showed "Electronic Stability System Fault". Read fault code C012104. Data stream indicated "valve relay voltage" at only 8.5V (normal >11V). External power supply tested normal. Fault traced to aging relay contacts inside the HCU causing excessive contact voltage drop. Replaced ESP hydraulic control unit (integrated IBooster system). Performed zero-point calibration of yaw rate sensor and longitudinal acceleration sensor. ESP function returned to normal.
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.