B1FC713 Relay Normal MOS Open Circuit Failure Technical Description

Fault Depth Definition

DTC B1FC713 (Relay Normal MOS Open Circuit Failure) is a critical diagnostic code in the vehicle Power Control and Power Management System. This code defines the monitoring result of the Control Unit regarding the switch status of specific high-voltage or large current transmission paths. In power electronic architecture, "MOS" refers to Metal-Oxide-Semiconductor Field-Effect Transistor (Metal-Oxide-Semiconductor Field-Effect Transistor), whose core function is to serve as an electronic switch to real-time feedback the physical connection status between the motor and power supply, and control current conduction and interruption.

The semantic analysis of this fault code indicates: the system detects that the MOS component inside the Relay is in an "Open Circuit Failure" state; specifically, when the control system issues an ON command, the actuator fails to establish an effective low-impedance loop. This typically means that within specific load paths or power distribution logic, the physical on-off function of the switch element is lost, causing the Control Unit unable to correctly construct the expected electrical feedback loop.

Common Fault Symptoms

When DTC B1FC713 is recorded or lit up in a diagnostic tool, the driver and vehicle system may exhibit the following perceivable abnormal phenomena:

• Ignition & Start Anomalies: The vehicle cannot start normally, or the Starting Motor responds sluggishly; the Starter Motor fails to engage, causing the vehicle to lose mobility capability under specific operating conditions.
• Dashboard Warning Information: The onboard diagnostic system sends high-priority signals to the instrument cluster via the CAN bus. Drivers will see Powertrain-related warning lights light up (e.g., Powertrain Warning Light). Some models may be accompanied by prompt icons related to battery or charging systems.
• High Voltage Cutoff Logic Activated: For safety protection, the Control Unit may trigger High-Voltage Interlock Failure logic, forcing the cut-off of driving motor power supply. The vehicle may suddenly lose power during dynamic driving or enter Limp Home Mode (Limp Home Mode).
• Intermittent Electrical Faults: When load fluctuation is large, due to poor MOS contact, short-term reset of vehicle electronic systems, abnormal lighting, or interruption of auxiliary power output may occur.

Core Fault Cause Analysis

Addressing DTC B1FC713 and associated "Possible Causes" description, technical failure logic at the technical level is summarized into the following three independent dimensions:

• Hardware Component

  • This is the most direct physical source for "Starting Battery Failure". The MOS tube itself may suffer breakdown or open circuit degradation due to long-term high-current thermal stress; the Relay's electromagnetic coil or contacts may undergo oxidation or ablation, leading to mechanical failure to close. Additionally, power semiconductor devices inside the power module associated with the "Starting Battery" may also age and fail, resulting in infinite on-resistance.

• Wiring & Connectors

  • Although the core is MOS open circuit, external physical connections may exacerbate failure determination. High-voltage harnesses or low-voltage control wires may have broken wires, insulation layer damage leading to ground short circuits, or loose/corroded connector terminals causing high contact resistance. This high-impedance state will be misjudged by the controller as internal open circuit failure.

• Controller

  • The logic operation part inside the Control Unit may detect signals inconsistent with expectations. For example, the drive signal voltage for the MOS gate is normal, but no expected load feedback current or voltage change is received on the power side. The Controller's ADC sampling module may also incorrectly mark the open circuit state due to threshold deviation, which belongs to a control strategy level judgment abnormality.

Technical Monitoring & Trigger Logic

The determination of this fault code is not static, but based on dynamic real-time calculation by the system under specific operating conditions:

• Monitoring Target

  • The system mainly monitors the conduction voltage drop across the MOS tube and the real-time current signal flowing through the Relay. The Control Unit continuously compares the "Command Voltage" and "Feedback Current" logical relationship, while also monitoring voltage fluctuation characteristics in the loop.

• Numerical Range Condition

  • During monitoring, after the Controller issues an $ON$ drive command, the expected loop current should immediately rise to the working interval. If the measured current stays near the $0A$ level for a long period, and the system detects leakage current under off-state abnormally increased or feedback signals are completely missing, the system will judge it as open circuit. Although specific thresholds are determined by calibration, usually this monitoring occurs in a stable state after load establishment to ensure exclusion of transient interference.

• Trigger Condition

  • The specific conditions for fault determination mainly concentrate on the Driving Motor or System Power-On Initialization phases. When the Relay receives an open/close command, the system needs to confirm the physical connection is established within a very short time (e.g., within $10ms$). If expected electrical feedback is not detected within this time window, and repeated monitoring confirmation shows no recovery, the system will formally write DTC B1FC713 and store fault frame data.