What Does B1793 Mean?
B1793 indicates the Body Control Module (BCM) has detected a short to ground condition in the automatic headlamp sensor circuit. This sensor, often called a photocell or ambient light sensor, monitors external light levels to automatically control headlamp operation. The BCM has detected that the sensor signal wire is shorted to chassis ground, causing an abnormally low or zero voltage reading instead of the expected variable voltage based on light conditions.
Common Causes
45%
Damaged or chafed wiring harness causing signal wire to contact ground, typically near sharp edges, body seams, or mounting brackets
30%
Failed autolamp sensor with internal short to ground within the sensor housing or connector pins
15%
Corroded or moisture-damaged connector at the autolamp sensor causing short circuit path to ground
10%
BCM internal circuit fault or damaged BCM connector pins causing false ground detection
Diagnostic Steps
1
Step 1: Perform visual inspection of autolamp sensor (typically mounted on dashboard near windshield or on top of instrument cluster) and its wiring harness. Look for physical damage, chafing, pinched wires, or moisture intrusion at connector.
2
Step 2: Disconnect the autolamp sensor connector and inspect terminals for corrosion, pushed-back pins, or moisture. Using a digital multimeter set to resistance mode, measure resistance between the signal wire terminal (sensor side disconnected) and chassis ground. Should read infinite resistance (OL); if continuity exists, trace wiring harness for short to ground.
3
Step 3: With sensor still disconnected, measure resistance across the sensor terminals. Compare readings to manufacturer specifications (typically 1-10k ohms variable with light exposure). If sensor shows near-zero resistance to ground pin or between signal and ground, replace sensor.
4
Step 4: If wiring and sensor test normal, inspect BCM connector for damage, corrosion, or terminal issues. Check for aftermarket accessories or repairs that may have compromised the circuit integrity near BCM location.
5
Step 5: Reconnect all components, clear codes, and verify repair by monitoring autolamp sensor data with scan tool while varying light conditions (cover sensor, shine light on it). Voltage should vary between approximately 0.5-4.5 volts depending on light intensity without code returning.
Estimated Repair Cost
$75 - $400
Parts + labor, varies by vehicle and location
B1793 indicates the Body Control Module (BCM) has detected a short to ground condition in the automatic headlamp sensor circuit. This sensor, often called a photocell or ambient light sensor, monitors external light levels to automatically control headlamp operation. The BCM has detected that the se...
The most common cause of B1793 (Autolamp Sensor Input Circuit Short To Ground) is: Damaged or chafed wiring harness causing signal wire to contact ground, typically near sharp edges, body seams, or mounting brackets
Typical repair costs for B1793 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. The autolamp feature will not function automatically, requiring manual headlamp operation. This is a convenience feature failure with no direct safety impact if driver manually operates lights appropriately, though it may result in unintentional daytime headlamp operation or failure to automatically illuminate at dusk.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1793 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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