What Does B1790 Mean?
This code indicates the Body Control Module (BCM) has detected a malfunction in the automatic headlamp sensor circuit, which controls when headlights turn on automatically based on ambient light levels. The BCM is receiving an out-of-range signal, no signal, or an implausible reading from the photocell sensor typically mounted on the dashboard. This affects the vehicle's automatic lighting system functionality.
Common Causes
45%
Faulty or degraded ambient light sensor/photocell
30%
Damaged, corroded, or loose wiring/connector at light sensor
15%
Failed or corroded body control module connector pins
10%
Internal BCM failure or software corruption
Diagnostic Steps
1
Step 1: Locate the ambient light sensor (typically on top of dashboard near windshield or integrated into rearview mirror base) and perform visual inspection for physical damage, debris covering sensor, or obvious connector issues
2
Step 2: Using a digital multimeter, check for reference voltage (typically 5V) and ground at the sensor connector with key on. Verify voltage remains stable and ground has less than 0.1V resistance to battery negative
3
Step 3: Measure the sensor output signal voltage while covering and uncovering the sensor with your hand or dark cloth. Signal should vary between approximately 0.5V (bright) and 4.5V (dark) or vice versa depending on manufacturer
4
Step 4: If voltage readings are correct at sensor, inspect wiring harness from sensor to BCM for chafing, pinches, or water intrusion. Pay special attention to areas where harness passes through firewalls or door jambs
5
Step 5: Check BCM connector terminals for corrosion, backed-out pins, or moisture. Clean and apply dielectric grease if corrosion is present
6
Step 6: If all wiring and voltage checks pass, replace the ambient light sensor and clear codes. If code returns immediately, suspect BCM internal fault and verify with known-good module or manufacturer diagnostic software
Estimated Repair Cost
$75 - $400
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected a malfunction in the automatic headlamp sensor circuit, which controls when headlights turn on automatically based on ambient light levels. The BCM is receiving an out-of-range signal, no signal, or an implausible reading from the photoc...
The most common cause of B1790 (Autolamp Sensor Input Circuit Failure) is: Faulty or degraded ambient light sensor/photocell
Typical repair costs for B1790 range from $75 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving. Automatic headlamps will not function properly, requiring manual operation of headlights. Repair can be scheduled at convenience, but ensure headlights are manually turned on when needed to maintain visibility and comply with traffic laws.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1790 to identify the root cause.
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Quick Info
Category
Body
System
Automatic Lighting System
Difficulty
Type
Manufacturer
Recommended Tools
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