What does it mean?
B1791 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Autolamp Sensor Input Circuit Open .
Symptoms
- Headlights do not switch on automatically at dusk or in a tunnel, or stay on all the time in daylight
- Instrument cluster and display no longer dim automatically
- Automatic lighting appears to work randomly, or the light switch works only in manual positions
- Warning message about the light sensor or automatic lighting on some vehicles
- Problem often starts right after windscreen replacement or dash work
Likely causes
- Sensor connector on top of the dash unplugged or left loose after windscreen, dash or speaker work
- Broken wire in the dash loom where it runs to the sensor, often at a bend or a clip
- Corroded or backed-out terminals in the sensor connector
- Ambient light sensor failed internally, with an open element
- Loss of the sensor's supply or ground because of a chafed wire or a poor ground point
- Damage caused by a windscreen replacement, when the loom is pulled or the sensor is not reconnected
- Body or lighting control module input stage failure
How to fix it
- Read the codes and check the live light level value on the scan tool
- Cover the sensor with your hand and then expose it to bright light; a healthy circuit shows the value change
- Lift the sensor from its dash aperture and inspect the connector for corrosion, loose fit and bent pins
- Check the supply and ground at the sensor connector with the ignition on
- Test the signal wire between the sensor and the module for continuity and for an open in the dash loom
- Repair the wiring or replace the light sensor if the circuit is proven good up to its connector
- Replace the lighting or body control module only after wiring and sensor have been ruled out