B1790 — Autolamp Sensor Input Circuit Failure #

Note: The code description may vary depending on the vehicle manufacturer.

• Autolamp Sensor Input Circuit Failure (Ford, Lincoln, Mercury)
• Automatic Headlamp Control Sensor Circuit Malfunction (Mazda)
• Light Sensor Signal Error (Toyota, Lexus)

Summary of DTC B1790 #

DTC B1790 is a diagnostic trouble code that indicates a problem in the autolamp sensor input circuit. This circuit is responsible for detecting ambient light levels and signaling the vehicle’s body control module or lighting control unit to automatically turn on or off the exterior lights. The autolamp sensor—sometimes called a light sensor or sunload sensor—can be found on the dashboard, near the windshield, or integrated into the rearview mirror, depending on the OEM. When the circuit fails, the vehicle’s automated lighting system may not function as intended, potentially impacting night driving safety or regulatory compliance. Addressing this code typically involves electrical diagnostics and, occasionally, part replacement.

Common Symptoms for Code B1790 #

When B1790 is present, you’ll notice changes in how your vehicle’s automatic lighting behaves. These symptoms may be subtle at first but can become more pronounced, affecting both convenience and safety. Recognizing these signs can help you diagnose the issue before it becomes a bigger problem:

• Automatic headlights do not turn on or off with changing ambient light
• Headlights may remain on during daylight or fail to activate at dusk/night
• Warning light or message related to exterior lighting on the dashboard
• Manual headlight control remains functional
• Occasional flickering or erratic operation of exterior lights

Probable Causes for B1790 (Most to Least Common) #

Several underlying issues can trigger the B1790 code. The most frequent are electrical in nature, but sensor failure or module issues are also possible. Here’s how they typically rank:

1. Faulty autolamp (light) sensor — The sensor itself can fail due to internal defects or exposure to heat/moisture.
2. Damaged wiring or connector — Broken, corroded, or shorted wires/connectors in the sensor circuit are common.
3. Poor ground or power supply — Insufficient voltage or a bad ground connection can interrupt sensor function.
4. Body Control Module (BCM) or Lighting Control Module fault — Rare, but possible if the control unit misinterprets or fails to process sensor input.
5. Incorrect sensor installation or aftermarket modifications — Non-OEM sensors or changes to the dashboard can disrupt sensor readings.

Step-by-Step Diagnostic Process for B1790 #

Diagnosing B1790 requires a systematic approach to isolate the fault. Accurate troubleshooting helps prevent unnecessary part replacement and ensures your repair is effective:

• Verify the code with a scan tool and clear it to see if it returns after a short drive cycle.
• Locate the autolamp sensor using OEM service information (dashboard, rearview mirror, etc.).
• Visually inspect the sensor and its immediate wiring for signs of damage, corrosion, or disconnection.
• Check for proper voltage supply and ground at the sensor connector with a multimeter.
• Test the sensor’s output using manufacturer-specified procedures—often involves measuring resistance or voltage change under varying light conditions.
• If wiring and sensor test good, check for related BCM or lighting module software updates or internal faults.
• Substitute with a known-good sensor or perform a “wiggle test” on the harness to rule out intermittent faults.
• Re-scan and check for code recurrence. Document findings and repair steps.

Possible Repairs for B1790 #

Once the root cause is identified, repairs should target the specific fault to ensure lasting results. Here are the most common fixes, mapped to their respective causes:

• Replace the faulty autolamp/light sensor (if sensor fails tests).
• Repair or replace damaged, corroded, or shorted wiring/connectors in the sensor circuit.
• Clean and secure ground connections for the sensor and BCM/lighting module.
• Reinstall or correctly position the sensor if improperly mounted or blocked (e.g., by dash covers or debris).
• Update or reprogram the BCM or lighting control module if a software-related issue is found (OEM-specific).
• Replace BCM or lighting control unit as a last resort only if module failure is confirmed.

Is It Safe to Drive With Code B1790? #

Driving with code B1790 is generally safe, but with limitations. The vehicle’s manual lighting controls will still function, so you can turn your headlights on or off as needed. However, because the automatic headlight feature is disabled, you must be vigilant about fading light conditions. Forgetting to manually activate headlights at dusk or in tunnels may compromise visibility and safety. If your vehicle is operated in areas where automatic lighting is a legal requirement, prompt repair is advised.

Vehicles Most Commonly Affected by B1790 #

While B1790 can appear in many modern vehicles equipped with automatic lighting, it is most frequently seen in certain models and brands, especially those using similar modules or sensor designs. The following vehicles are often reported:

• Ford F-Series (F-150, F-250, etc.)
• Ford Explorer, Taurus, Fusion
• Lincoln MKX, MKZ
• Mazda 3, Mazda 6 (with auto lights)
• Toyota Camry, Corolla, Avalon (selected years)
• Lexus RX, ES (with rain/light sensor)

Common Mistakes & Best Practices for B1790 #

When troubleshooting B1790, some errors can lead to wasted time or unnecessary expenses. Keep these best practices in mind:

• Do not replace the sensor before confirming power, ground, and wiring integrity.
• Always check for TSBs (Technical Service Bulletins) or recalls related to sensor or BCM issues for your vehicle.
• Use OEM parts for sensor replacement to avoid compatibility issues.
• Inspect for aftermarket tint or dash covers that can block the sensor’s light path.
• Document all findings and settings before and after repair for future reference.

Quick FAQ for B1790 #

Is B1790 covered under warranty?
Replacement of the autolamp sensor or repairs to the lighting control circuit may be covered under your vehicle’s basic or emissions warranty. Check with your dealer and have your VIN ready for confirmation.

Can a dead battery or jump-start cause code B1790?
While rare, a low voltage event or improper jump-start can disrupt sensitive circuits or modules, occasionally causing a false B1790 code. Always clear codes and retest after any electrical service.