Ambient Light Sensor Circuit Failure
Code B1790 indicates a fault in the ambient light sensor (autolamp sensor) circuit of the vehicle's automatic lighting system. This sensor, typically located on the dashboard or windshield, detects outside light conditions to automatically turn the headlights on or off. The body control module (BCM) has detected an out-of-range, intermittent, or missing input signal from the sensor, preventing the autolamp system from functioning correctly.
1. Commonly Affected Makes
2. Most Common Causes
- Damaged or degraded ambient light sensor (photocell) due to prolonged sun exposure
- Open, shorted, or high-resistance wiring in the sensor circuit
- Corroded, oxidized, or damaged sensor connector pins
- Faulty body control module (BCM) or associated lighting module
- Physical obstruction of the sensor by stickers, objects, or dirt buildup
- Windshield tint or repair that alters light transmission to the sensor
3. Symptoms You Will Notice
- Automatic headlights not turning on or off as expected
- Headlights staying on constantly or not activating in low light
- Dashboard lights or interior illumination behaving erratically
4. Step-by-Step Diagnosis
Inspect the ambient light sensor for physical damage, dirt, or obstructions
Check the sensor connector for corrosion, bent pins, or poor contact
Test the sensor circuit wiring for opens, shorts, or high resistance using a multimeter
Verify sensor operation by measuring its output signal under varying light conditions
If wiring and sensor are okay, suspect a BCM fault and perform module diagnostics
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive, but automatic headlight functionality will be impaired. You may need to manually operate the headlights.
How much does it cost to fix?▼
Repair costs typically range from $100 to $400, depending on whether the sensor, wiring, or BCM needs replacement.