B1791 – Autolamp Sensor Input Circuit Open

The ODBII code B1791 indicates that there is an open circuit in the autolamp sensor input, which means the vehicle’s system is not receiving a signal from the autolamp sensor that detects ambient light levels.

The severity of ODBII code B1791 is moderate.

Common Causes For: B1790 Autolamp Sensor Input Circuit Failure

• Faulty autolamp sensor
• Open circuit in wiring
• Corroded or damaged connectors
• Short circuit in wiring
• Faulty body control module (BCM)
• Poor ground connection
• Disconnected or loose wiring harness

Common misdiagnosis for B1791 Autolamp Sensor Input Circuit Open

• Faulty headlight switch
• Defective body control module (BCM)
• Wiring issues or shorts in the circuit
• Malfunctioning autolamp sensor
• Incorrectly diagnosed battery or charging system problems
• Issues with the vehicle’s lighting system or bulbs
• Faulty connectors or terminals in the circuit

Common Troubleshooting Steps

The OBD-II code B1791 refers to an issue with the autolamp sensor input circuit, indicating that there is an open circuit in the sensor input. Here are some common troubleshooting steps to diagnose and resolve this issue:

• Check the Vehicle’s Manual: Refer to the vehicle’s service manual for specific information regarding the autolamp sensor and its circuit. This can provide valuable insights into the wiring diagrams and component locations.
• Visual Inspection:
• Inspect the wiring and connectors associated with the autolamp sensor for any signs of damage, corrosion, or loose connections.
• Look for frayed wires, broken connectors, or any other physical damage that could cause an open circuit.
• Test the Autolamp Sensor:
• If accessible, test the autolamp sensor itself using a multimeter. Check for continuity and ensure that the sensor is functioning correctly.
• If the sensor is faulty, replace it.
• Check the Circuit:
• Use a multimeter to check for voltage and ground at the sensor connector. Ensure that the circuit is receiving the proper voltage and that the ground is intact.
• If there is no voltage or ground, trace the wiring back to the power source or ground point to identify any breaks or shorts.
• Inspect Related Components:
• Check any related components, such as the Body Control Module (BCM) or any fuses associated with the autolamp system. A blown fuse could cause an open circuit.
• Ensure that the BCM is functioning properly, as it may be responsible for processing the input from the autolamp sensor.
• Clear the Code:
• After performing the necessary repairs, clear the trouble code using an OBD-II scanner and take the vehicle for a test drive to see if the code returns.
• Consult Technical Resources:
• If the issue persists, consult technical resources or forums specific to your vehicle make and model for additional troubleshooting tips or common issues related to the B1791 code.
• Professional Diagnosis:
• If you are unable to resolve the issue, consider seeking help from a professional mechanic or automotive technician who has experience with electrical systems and OBD-II diagnostics. By following these steps, you should be able to identify and resolve the issue related to the B1791 code.