B1791: Autolamp Sensor Input Circuit Open

Definition

The OBD2 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

Common causes

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 misdiagnoses

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

Troubleshooting steps

1. 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
2. Visual Inspection
3. Inspect the wiring and connectors associated with the autolamp sensor for any signs of damage, corrosion, or loose connections
4. Look for frayed wires, broken connectors, or any other physical damage that could cause an open circuit
5. Test the Autolamp Sensor
6. If accessible, test the autolamp sensor itself using a multimeter. Check for continuity and ensure that the sensor is functioning correctly
7. If the sensor is faulty, replace it
8. Check the Circuit
9. 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
10. If there is no voltage or ground, trace the wiring back to the power source or ground point to identify any breaks or shorts
11. Inspect Related Components
12. 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
13. Ensure that the BCM is functioning properly, as it may be responsible for processing the input from the autolamp sensor
14. Clear the Code
15. 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
16. Consult Technical Resources
17. 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
18. Professional Diagnosis
19. 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

Definition

Common causes

Common misdiagnoses

Troubleshooting steps

1. Check the Vehicle's Manual

2. Visual Inspection

3. Inspect the wiring and connectors associated with the autolamp sensor for any signs of damage, corrosion, or loose connections

4. Look for frayed wires, broken connectors, or any other physical damage that could cause an open circuit

5. Test the Autolamp Sensor

6. If accessible, test the autolamp sensor itself using a multimeter. Check for continuity and ensure that the sensor is functioning correctly

7. If the sensor is faulty, replace it

8. Check the Circuit

9. 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

10. If there is no voltage or ground, trace the wiring back to the power source or ground point to identify any breaks or shorts

11. Inspect Related Components

12. 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

13. Ensure that the BCM is functioning properly, as it may be responsible for processing the input from the autolamp sensor

14. Clear the Code

15. 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

16. Consult Technical Resources

17. 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

18. Professional Diagnosis

19. 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