Automatic headlights stopped working: sensor or setting

Automatic headlights use a light sensor, usually mounted at the top of the windscreen near the mirror, often in the same housing as the rain sensor.

When they stop working, the cause is frequently not a fault at all.

Related: headlight bulbs and condensation.

Check these three things first

1. The switch is in AUTO

Obvious, and it accounts for a real share of these complaints. The switch gets knocked, or someone else drove the car.

2. Something is covering the sensor

The sensor sits behind the windscreen at the top, and anything in front of it blinds it:

• A sunshade or window sticker.
• A dashcam or phone mount in the wrong position.
• A parking permit or toll tag.
• Heavy tint applied over the sensor area.
• Ice or snow on the outside.

Look at the area around the mirror mount. If something is in front of the sensor, the system is working correctly and being lied to.

Dashcam installation done right.

3. The windscreen was replaced

A very common cause, and one people never connect.

The sensor bonds to the glass through a gel pad. If the windscreen was replaced and the sensor was refitted without a fresh pad, or with an air bubble underneath, it cannot see properly.

If automatic lights or wipers stopped working after a windscreen replacement, that is why.

Windscreen chip repair vs replacement and rain sensing wipers not reacting.

Behaviour that is not a fault

Sensitivity varies between vehicles, and some are noticeably later to switch on than owners expect.

Many systems switch on in tunnels and under bridges, which is intended.

Some are deliberately less sensitive in daylight to avoid switching on under every overhanging tree.

On many vehicles the threshold is adjustable through coding — if the behaviour is merely not to your taste rather than absent, that is the route: manufacturer coding: VAG, BMW, Mercedes and coding, programming and flashing explained.

When it is a genuine fault

The sensor

Fails outright or drifts with age. Usually part of a combined rain and light sensor module.

If automatic wipers also stopped, the shared module is the likely cause, and that narrows it considerably.

Connector and wiring

At the top of the windscreen, disturbed by any work on the mirror, the headliner or the glass.

The body control module

Commands the lights based on the sensor’s input. Faults here usually affect other lighting functions too: BCM body control module diagnostics.

Low system voltage

Comfort functions are among the first to be shed when the battery is low. A car that disables convenience features intermittently is telling you about its battery: P0560 to P0563 system voltage faults.

The safety angle

Automatic lights create a habit of not thinking about lights. When they fail, the failure mode is driving without lights at dusk while believing they are on.

The daytime running lights make this worse, because from inside the car the dashboard is lit and the road ahead looks illuminated — while the tail lights are off and you are effectively invisible from behind.

Check your tail lights are on in poor visibility rather than assuming. It is the specific risk this fault creates.

Dashboard warning lights priority and LED conversion legality and optics.

In what order

1. Check the switch is in AUTO.
2. Look for anything covering the sensor at the top of the windscreen.
3. Ask whether the windscreen was replaced recently.
4. Check whether automatic wipers also failed — a shared module.
5. Check system voltage.
6. Read body module codes where accessible.
7. Until fixed, switch lights on manually in poor visibility.

Try it on the free tier of the app — pair an ELM327 adapter and check system voltage before suspecting a comfort module. Features that switch themselves off are often a power symptom rather than a fault.