C1B8704 - C1B8704 EPS Fault - Torque Sensor (Torque Sensor Internal Fault)

Detailed Fault Definition

C1B8704 Electronic Power Steering System (EPS) DTC specifically points to internal integrity failure of the torque sensor component. In modern automotive chassis control systems, the electronic power steering controller reads real-time data from the torque sensor to precisely calculate driver's driving intent and apply corresponding assist torque. The core meaning of this fault code is "Internal Fault", meaning the diagnostic logic confirms the fault point is located within the physical or electronic components of the sensor itself, rather than external wiring open circuit or short circuit.

Under this definition, the EPS Control Unit (EPC/ECU) verifies the credibility of torque signals. When the system detects that the analog voltage or digital pulse signal from the torque sensor exceeds preset reference feature values and cannot be resolved via software calibration logic, it will be judged as sensor internal component damage or performance drift. This fault directly relates to the feedback loop of the power steering system; if not handled promptly, it will lead to the controller being unable to correctly parse steering input quantity, thereby affecting closed-loop control strategy of power distribution.

Common Fault Symptoms

When C1B8704 fault code is activated and stored in controller memory, vehicle driving experience will show the following significant physical feedback and instrument indicators:

• Abnormal Steering Feel: Drivers will feel a sudden increase in steering wheel heaviness, losing the lightness brought by electronic power assistance, especially when torque input on steering wheel (Torque Input) is large or in specific speed ranges.
• Instrument Warning Light Illumination: EPS warning light or vehicle power steering assist system indicator light on the combination instrument panel will stay lit, indicating the system detects key sensor data failure.
• Driving Stability Feedback: Under high-speed driving or emergency avoidance conditions, due to interruption of assist logic, steering response may show delay or uneven force phenomena.
• DTC Storage and Freeze Frame: On-board diagnostic system will record this DTC information, and may trigger relevant safety degradation modes (Safe Mode), limiting electronic assistance functions to ensure independent availability of mechanical steering functions.

Core Fault Cause Analysis

Regarding causes for C1B8704, based on raw data and system architecture, mainly can be summarized into following three technical dimensions:

• Hardware Component Failure: Torque sensor internal circuit damage. This is the failure point explicitly indicated in title, involving permanent physical damage to Hall elements inside sensor, potentiometers or integrated circuits (IC), causing inability to output accurate steering angle and torque data.
• Controller Logic Failure: Electronic Power Steering Controller (EPS Controller) itself abnormal. According to possible fault cause description, controller as data processing hub, if its internal diagnostic module (Self-Diagnosis Module) or processor appears logic operation errors, internal storage area damage or voltage reference source failure when processing sensor signals, will also directly generate C1B8704 such fault code pointing to controller.
• Physical Connection and Signal Integrity: Although title mainly points to internal faults, connector oxidation, contact resistance too large or poor grounding may cause controller to sample unstable analog voltage signals (Signal Integrity Failure), triggering controller's internal redundant protection logic, thus judging as component internal abnormality.

Technical Monitoring and Trigger Logic

Electronic power steering system has extremely high requirements for torque sensor real-time nature; specific monitoring logic for triggering this fault code is as follows:

• Monitoring Target: Mainly monitors analog voltage signals (Analog Voltage Signal) from torque sensor or torque command values in digital communication bus (CAN Bus) data packets.
• Numerical Range and Threshold Judgment: Controller internally sets strict normal signal windows (Normal Operation Window). When amplitude, frequency characteristics or duty cycle of sensor output signal continuously exceeds this calibration range, system judges as out-of-limit. Additionally, controller will perform consistency verification, comparing logical rationality between torque sensor input values and other associated sensors (like vehicle speed signals). If signal mutation or missing, and duration exceeds preset diagnostic time window, fault occurrence is judged.
• Specific Condition Trigger: This fault usually triggers under dynamic monitoring state when engine running, transmission in gear and driver exists steering wheel turning operations. During static idle if no reasonable steering input detected, system may temporarily not report fault or just record as history fault; once enter driving dynamic conditions, controller executes real-time sampling, once data stream does not fit logic model, immediately light fault light and record C1B8704 to prompt maintenance.
• Diagnostic Strategy: Controller will adopt "multiple sampling comparison" strategy, confirming fault is not instantaneous interference, but continuous hardware failure, thus confirming internal permanent fault rather than false alarm caused by electromagnetic interference.