What Does B1791 Mean?
This code indicates the Body Control Module (BCM) has detected an open circuit in the automatic headlamp sensor input circuit. The autolamp sensor, typically mounted on the dashboard near the windshield, monitors ambient light levels to automatically control exterior lighting. An open circuit means the BCM is not receiving any signal from the sensor, which could be due to wiring damage, connector issues, or sensor failure.
Common Causes
40%
Damaged or corroded wiring/connectors in the autolamp sensor circuit
30%
Failed autolamp/photocell sensor
20%
Broken or disconnected sensor connector
10%
BCM internal circuit failure or poor ground connection
Diagnostic Steps
1
Step 1: Visually inspect the autolamp sensor (typically located on top of dashboard near windshield) for physical damage, disconnected connectors, or obvious wiring issues
2
Step 2: Using a digital multimeter, check for continuity between the sensor connector and BCM connector terminals on the signal circuit wire with ignition off - should read less than 5 ohms resistance
3
Step 3: Check for proper voltage supply at the sensor connector with ignition on - typically should see 5V or 12V reference voltage depending on manufacturer specifications
4
Step 4: If wiring and voltage are correct, disconnect the sensor and measure its resistance across the appropriate terminals while covering/uncovering the sensor with your hand - resistance should change with light level changes
5
Step 5: Check BCM ground circuits for continuity to chassis ground - should read less than 1 ohm resistance
6
Step 6: If all wiring and grounds test good but sensor fails resistance test, replace the autolamp sensor and clear codes, then verify automatic headlamp operation in varying light conditions
Estimated Repair Cost
$75 - $350
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected an open circuit in the automatic headlamp sensor input circuit. The autolamp sensor, typically mounted on the dashboard near the windshield, monitors ambient light levels to automatically control exterior lighting. An open circuit means ...
The most common cause of B1791 (Autolamp Sensor Input Circuit Open) is: Damaged or corroded wiring/connectors in the autolamp sensor circuit
Typical repair costs for B1791 range from $75 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving. The automatic headlamp feature will not function, requiring manual control of exterior lights. This is a convenience issue rather than a safety hazard, though drivers must remember to manually operate lights in low-light conditions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1791 to identify the root cause.
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Quick Info
Category
Body
System
Exterior Lighting Control System
Difficulty
Type
Manufacturer
Recommended Tools
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