B1791: Auto Headlight Sensor Circuit Open
The B1791 code indicates that the Body Control Module (BCM/GEM) has detected an open circuit in the input signal from the automatic headlight sensor (ambient light sensor). This sensor, typically located on the dashboard near the windshield, sends a variable voltage signal to the BCM based on ambient light levels. An open circuit prevents the BCM from accurately determining outside light conditions, disabling the automatic headlight function.
1. Commonly Affected Makes
2. Most Common Causes
- Faulty automatic headlight sensor (daylight/ambient light sensor) with internal failure
- Open circuit in the signal, ground, or reference wiring of the sensor (cut, disconnected)
- Damaged, loose, or corroded/retracted terminals in the sensor connector
- Internal failure in the Body Control Module (BCM/GEM) at the sensor input pin
3. Symptoms You Will Notice
- Automatic headlights do not turn on or off as expected
- Headlights may stay on constantly or fail to activate in low light
4. Step-by-Step Diagnosis
Scan for codes and verify B1791 is present
Visually inspect the ambient light sensor and its connector for damage, corrosion, or loose terminals
Test the sensor's signal circuit for continuity and proper voltage reference
If wiring and sensor check out, test the BCM input pin for proper operation
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive, but automatic headlights will not function. You can still operate headlights manually. The issue is moderate and should be repaired soon for convenience and safety.
How much does it cost to fix?▼
Repair costs typically range from $100 to $300, depending on whether the sensor or wiring needs replacement. BCM replacement is more expensive, often $500-$1000.