B1791

Autolamp Sensor Input Circuit Open

Body Chassis/Safety Lighting Control Circuit 🟢 Low — Fix at next service ✅ Safe to Drive
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What This Actually Means

In plain language — no jargon

The autolamp sensor circuit has an open connection, meaning the wire or sensor is broken and the ECU cannot read the signal. It's like trying to turn on a light with a cut wire—the power can't complete the circuit.

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Symptoms You May Notice

3 known symptoms for this code
Dashboard lights or exterior lamps not functioning properly
Illumination control system malfunction or erratic behavior
Check Engine light or body control module warning illuminated
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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECU monitors voltage levels from the autolamp sensor circuit to detect ambient light conditions and control automatic headlight operation. When the circuit is open, the ECU detects zero or no valid voltage signal where it expects a variable analog input.

Voltage & Parameter Thresholds

ParameterNormal RangeFault Condition
Sensor voltage signal 0.5V to 4.5V (variable with light intensity) Below 0.1V or no signal detected (open circuit)
Circuit continuity Resistance within spec (typically <10 ohms) Infinite resistance or open detected
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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex
1
Wire harness and connectors
Inspect wiring from sensor to ECU for breaks, corrosion, or loose connectors and reseat or repair as needed.
2
Autolamp sensor
Test sensor with multimeter for continuity; replace if open or damaged.
3
Body control module connector
Clean corroded pins on the BCM or lighting control module connector using contact cleaner and a soft brush.
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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code B1791 is a low-severity fault. Your vehicle is generally safe to drive to a workshop for diagnosis. However, do not ignore it indefinitely — low-severity codes often indicate developing problems that become expensive if neglected. Book a diagnostic appointment within 2–4 weeks. If you notice any additional symptoms (rough running, power loss, unusual smells), treat it as higher priority.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
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How to Clear Code B1791

What happens after you fix the fault

Once the fault is repaired, B1791 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.