B1793

Code

Other B — Body

Autolamp Sensor Input Circuit Short To Ground

Brand: Other

Type: B — Body

Views: UK: 21 EN: 35 RU: 35

AI status

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Page language: EN

Causes

Damaged or pinched wiring in the sensor signal circuit causing contact with chassis ground
Corroded, loose or damaged sensor connector or terminal
Failed ambient light/autolamp sensor (internal short)
Faulty body control module (BCM) or autolamp control module
Aftermarket equipment or recent repairs that disturbed the wiring harness
Water intrusion in sensor or connector

Symptoms

Automatic headlamps not working (stays off or on)
Dashboard message or lamp indicating lighting system fault
Related lighting features (daytime running lights, auto-dim) not functioning correctly
DTC B1793 stored in BCM
Intermittent behavior when harness flexed near sensor or BCM

What to check

Read codes and freeze-frame data with a scan tool; confirm B1793 and check for related codes
Inspect sensor location and connector for corrosion, water intrusion, damage
Visually trace wiring from sensor to BCM for chafing, pinched sections or recent repairs
Backprobe the sensor signal wire and monitor live data while changing ambient light
Use a multimeter to check for continuity to ground on the signal wire with harness disconnected
Disconnect sensor and clear codes to see if code returns (helps isolate harness vs sensor)

Signal parameters

Sensor supply/reference: typically 5 V reference from BCM (verify with vehicle manual)
Signal voltage: varies with ambient light; typical ranges ~0.2–4.8 V (very dark ≈ low voltage, bright ≈ high voltage)
Short-to-ground symptom: signal at or near 0 V (ground) when it should vary
Short-to-battery symptom (for comparison): signal near battery voltage when shorted to Vbatt
Wiring resistance: low resistance to ground on signal wire when shorted (near 0 Ω)

Diagnostic algorithm

Connect a scan tool. Verify B1793 and record live data for the ambient light sensor input while changing light conditions.
Visually inspect the sensor, housing and connector for damage, moisture, corrosion or insect debris. Repair/clean connector if needed.
With ignition ON, backprobe the sensor connector: confirm reference voltage (if applicable), ground and the signal voltage. Compare to expected signal parameters.
If signal reads ~0 V (short to ground), disconnect the sensor and measure resistance between the signal wire and chassis ground. Low resistance indicates a short in the harness. If open/high resistance, suspect the sensor.
Wiggle the harness and connector while watching live data to check for intermittent shorts. Inspect sections where harness passes through body panels or near fasteners.
If harness short suspected, isolate by disconnecting intermediate connectors and retesting to narrow location. Repair damaged wiring (repair section, replace pin/connector, use proper sealing) and retest.
If harness tests OK with connector disconnected but sensor removed, replace the ambient light sensor and retest.
If symptom persists after sensor and wiring repair, inspect BCM input circuit. Verify power/ground to BCM and check for software updates or module faults. Replace BCM only after all wiring and sensor possibilities are eliminated.
After repair, clear codes and perform functional test across lighting modes and verify code does not return.

Likely causes

Wiring short to ground between the sensor and BCM (most common)
Corroded/contaminated connector at the sensor
Failed ambient light sensor
Module input driver fault (less common)

Fault status

⚠️ Status

Autolamp/ambient light sensor input circuit short to ground detected by control module. The module is receiving a low or grounded signal on the sensor input, disabling automatic lighting control until the circuit is repaired.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours