B1791 Code - Autolamp Sensor Input Circuit Open
Quick Answer
The Smart Junction Box (SJB) sends a voltage signal to the headlamp switch and the light sensor. Common causes include Faulty Light Sensor, Light Sensor harness is open or shorted, Light Sensor circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
- What it means
- B1791 indicates Autolamp Sensor Input Circuit Open.
- Common causes
- Common causes include Faulty Light Sensor, Light Sensor harness is open or shorted, Light Sensor circuit poor electrical connection.
- Severity
- High - diagnose this code as soon as possible.
- Can I drive?
- Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
Code B1791 Description
When the headlamp switch is placed in the AUTOLAMPS ON position, the voltage signal is routed to ground. When the SJB detects the headlamp switch in the AUTOLAMPS ON position, the SJB monitors the light sensor input. The light sensor determines the amount of light based on the input received from the ambient lighting conditions. When the ambient level has reached a point (determined by the internal programming of the SJB ), the SJB provides voltage to the headlamps, parking and license plate lamps. The Diagnostic Trouble Code (DTC) sets when the SJB detects an open from the light sensor input circuit.
What are the Possible Causes of the DTC B1791?
- Faulty Light Sensor
- Light Sensor harness is open or shorted
- Light Sensor circuit poor electrical connection
- Faulty Headlamp Switch
- Faulty Smart Junction Box (SJB)
How to Fix the DTC B1791?
Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.
What is the Cost to Diagnose the Code?
Labor: 1.0
To diagnose the B1791 code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.
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Information for Specific Makes
- B1791 Ford — Autolamp Sensor Input Circuit Open
- B1791 Lincoln — Autolamp Sensor Input Circuit Open
- B1791 Mercury — Autolamp Sensor Input Circuit Open
Frequently Asked Questions about B1791 Code
1. What does OBDII code B1791 mean?
Code B1791 indicates an "Autolamp Sensor Input Circuit Open," which suggests a problem in the circuit responsible for detecting ambient light levels for automatic headlight operation.
2. What are the common symptoms of code B1791?
Symptoms may include automatic headlights failing to turn on or off, headlights staying on during daylight, or a warning light on the dashboard.
3. What causes the B1791 code to appear?
The code is usually caused by a faulty autolamp sensor, damaged wiring, loose or corroded electrical connections, or a malfunctioning control module.
4. Can I still drive my vehicle with code B1791?
Yes, you can typically drive the vehicle safely, but the automatic headlight feature may not function correctly, requiring you to manually control the headlights.
5. How can I diagnose the issue causing code B1791?
Start by visually inspecting the autolamp sensor, wiring, and connectors for damage. Use a multimeter to check for continuity or voltage in the circuit, and verify the sensor's operation.
6. How do I fix the B1791 code?
Repairs may involve replacing the autolamp sensor, repairing damaged wiring, cleaning corroded connectors, or reprogramming/updating the control module if necessary.
7. Where is the autolamp sensor located?
The autolamp sensor is typically located on the dashboard near the base of the windshield or integrated into the rearview mirror assembly, depending on the vehicle.
8. How much does it cost to repair code B1791?
Repair costs vary, but replacing an autolamp sensor can cost $50–$200 for the sensor alone, plus labor costs if done by a professional.
9. Can a dead battery trigger code B1791?
A dead or weak battery is unlikely to directly cause this code, but electrical interruptions can sometimes lead to circuit-related issues. Testing the battery and charging system is recommended.
10. Do I need to clear code B1791 after repairs?
Yes, after fixing the issue, clear the code using an OBDII scanner and test the system to ensure the problem has been resolved.
Discuss the causes and fixes for B1791 OBDII Code
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