U101786 - U101786 EPS Signal Invalid (EPS Signal Value Invalid Fault)

U101786 EPS Signal Invalid (EPS Signal Value Invalid Fault) Technical Specification Document

Fault Severity Definition

DTC U101786 belongs to the diagnostic monitoring scope of the Electric Power Steering System (Electric Power Steering, EPS), with its core focus on "EPS Signal Value Invalid". In the vehicle electronic electrical architecture, this fault code represents the control unit detecting that communication signals from the EPS controller or internal feedback data lack logical validity. This not only implies an interruption in the transmission of steering assist signals but also reflects a failure in logical verification between systems (such as with Adaptive Cruise Control System ACC, Body Control Module BCM). When the control unit determines that the received EPS signal does not meet preset engineering thresholds or timing requirements, the system will be marked as "Invalid" to trigger protective strategies and prevent driving assistance functions from executing based on erroneous data.

Common Fault Symptoms

Based on the "Adaptive Cruise Control System Function Failure" defined in the original data as the core manifestation, combined with vehicle electrical network characteristics, owners and technicians can observe the following specific driving experiences or instrument feedback:

• Adaptive Cruise Control System (ACC) actively exits service; the dashboard may display ACC unavailable or system off warning icons.
• Steering assist data communication delay may occur during vehicle travel, leading to temporary unavailability of some high-order auxiliary functions.
• If the fault persists, the vehicle network management system will record DTC U101786 and maintain an active state within specific diagnostic cycles.
• The system may enter a limited operation mode, prioritizing basic steering mechanical performance while cutting off output of advanced features dependent on EPS signals.

Core Fault Cause Analysis

Regarding the trigger mechanism for DTC U101786, technical analysis is conducted from three dimensions: hardware components, wiring/connectors, and controller logic:

• Hardware Component Failure: Abnormality in the internal signal processing unit of the Electric Power Steering Control Unit (EPS Control Unit) prevents the generation of valid feedback signals; or the physical state of the EPS motor itself causes sensor output data to exceed the effective range.
• Wiring and Connector Connection Issues: The communication harness between the EPS controller and other control units (e.g., BCM, Gateway) exhibits high impedance, open circuit, or short circuit; poor contact at the EPS power connection terminals prevents normal digital signal transmission under specific voltage windows.
• Controller Logic Operation Abnormality: Incorrect software logic judgment within the EPS controller fails to correctly respond to network requests; or the controller does not execute the power-on self-check procedure normally, causing its state machine to fail sending a valid "Online" handshake signal to the bus.

Technical Monitoring and Trigger Logic

The generation of this fault code strictly follows a preset logic tree algorithm. The control unit monitors the validity of EPS signals in real-time under specific operating conditions. Only when all of the following conditions are met simultaneously will the system record U101786:

• Power Supply Voltage Threshold Judgment: The controller must operate within the effective working voltage range, i.e., supply voltage must be maintained between $9V \sim 16V$ to rule out signal misjudgment caused by power fluctuations.
• Timing Initialization Condition: After system startup, initialization delay is experienced; validity check of EPS signals begins after time exceeds $3s$.
• Bus Communication Status Monitoring: The Public CAN (Public CAN) network must remain online and not enter the busoff protection state, ensuring normal physical connectivity of the communication link.
• Mode and Interaction Logic Judgment:
  • Vehicle factory mode must be in an off state (Factory Mode Off), excluding interference from development test modes.
  • System has not received a de-power notice sent by the Body Control Module (BCM) (No BCM De-escalation Notice received), meaning EPS has not entered the expected power-off or safety release procedure.
  • If a service diagnostic code is detected, and that state continues for $3s$ without automatic clearing, the system will lock this fault logic and output U101786.