Autolamp Sensor Input Circuit Failure
Your car's automatic headlight sensor isn't sending proper signals to the computer, like a broken light detector that can't tell if it's day or night. The ECU can't control the auto-lamp function because the input circuit is open, shorted, or malfunctioning.
The ECU monitors voltage from the ambient light sensor circuit to determine surrounding light conditions and automatically control headlights. The sensor should provide a variable voltage signal that changes based on ambient light intensity. When the circuit opens, shorts to ground, or the sensor fails, the ECU detects an out-of-range signal and sets the fault code.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.5–4.5 volts (varies with light level) | Below 0.1V or above 4.8V; open/short circuit |
| Circuit Resistance | 1–10 kΩ (typical sensor range) | Infinite (open) or near 0Ω (short) |
Code B1790 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.
Once the fault is repaired, B1790 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.