What does AUTO on the headlight switch actually do?

It hands the decision to a light sensor, normally mounted behind the windscreen close to the rear-view mirror. The sensor reads ambient brightness and switches the sidelights and dipped beam on when it drops below a threshold, then off again when it rises. Most systems also react to tunnels and to prolonged shade. On many cars the same sensor pack runs the automatic wipers, which is why the two features usually arrive together.

Are automatic headlights good enough in fog?

No, and this is the one genuinely important thing to know about them. The sensor measures how bright it is, not how far you can see. Fog, dense spray behind lorries and falling snow are all commonly bright enough that the system leaves the headlamps off, at precisely the moment you most need to be seen. The Highway Code requires headlights whenever visibility is seriously reduced, and satisfying that is your responsibility, not the car's. Switch to dipped beam manually and add fog lamps if it is bad enough to justify them.

Do automatic headlights turn on the rear lights too?

When the system actually switches the headlamps on, yes — it brings up the full set including the rear position lamps and the registration plate lamp. The problem is the in-between state. In poor daylight the system often does nothing at all, leaving the car on daytime running lamps, which are front-only. So a car in AUTO in heavy rain can quite easily be showing nothing whatsoever at the back.

Is there an MOT check for automatic headlights?

No. Automatic headlamp control is not a testable item, there is no defect for its warning lamp, and no car is required to have it. The headlamps themselves are tested exhaustively for condition, colour, operation, aim and switching, but the tester operates them from the switch. A car whose light sensor has failed will still pass the MOT provided the headlamps work normally when switched on by hand.

Dashboard warning light

Automatic headlight light

Last updated 27 July 2026

Quick answer

Automatic headlamp control is active: a light sensor, usually behind the windscreen near the mirror, is deciding when to switch the headlamps on. Nothing is wrong. The limitation worth knowing is that the sensor measures brightness, not visibility, so it does not reliably switch the lights on in fog or spray.

What to do: No action needed
Can you drive?: Yes

Why it is green

Confirms something is switched on and working. Not a fault.

What turns it on

Turning the light switch to the AUTO position. The tell-tale confirms the mode is selected, not that the headlamps are currently lit — most cars show the separate green dipped beam symbol when the system actually turns them on.

What to do about it

Override it manually whenever visibility is poor in daylight. Fog, heavy spray, sleet and driving rain can all be bright enough that the sensor sees no reason to act, and you are then driving with unlit rear lamps in exactly the conditions where you most need them.
Check the dipped beam tell-tale, not the AUTO tell-tale, to know whether your headlamps are actually on.
Keep the area of windscreen in front of the sensor clear. A sticker, a dashcam mount, a heavily tinted strip or a badly done screen replacement can all confuse it.
If the headlamps flick on and off repeatedly under trees or through a tunnel entrance, that is the sensor working as designed rather than a fault.

Can you keep driving?

Yes

Yes. This is a mode indicator and not a fault of any kind. Treat it as a reminder that the car is making the decision for you, and take the decision back when the weather turns.

What causes it

The light switch is in the AUTO position
The car defaults to automatic on every start, as many now do
A rain and light sensor pack that also handles automatic wipers, sharing the same module

Does it fail an MOT?

No.

The MOT has nothing to say about automatic headlamp control. There is no defect for the tell-tale, no defect for the light sensor and no requirement for a car to have the feature at all. What the tester does check is the headlamps themselves: that they work, that they are the right colour, that the lens is sound, that the aim is within limits, and that the switch operates from the driver's seat. A car whose automatic system has failed but whose manual switch still works will pass.

Common questions

What does AUTO on the headlight switch actually do?: It hands the decision to a light sensor, normally mounted behind the windscreen close to the rear-view mirror. The sensor reads ambient brightness and switches the sidelights and dipped beam on when it drops below a threshold, then off again when it rises. Most systems also react to tunnels and to prolonged shade. On many cars the same sensor pack runs the automatic wipers, which is why the two features usually arrive together.
Are automatic headlights good enough in fog?: No, and this is the one genuinely important thing to know about them. The sensor measures how bright it is, not how far you can see. Fog, dense spray behind lorries and falling snow are all commonly bright enough that the system leaves the headlamps off, at precisely the moment you most need to be seen. The Highway Code requires headlights whenever visibility is seriously reduced, and satisfying that is your responsibility, not the car's. Switch to dipped beam manually and add fog lamps if it is bad enough to justify them.
Do automatic headlights turn on the rear lights too?: When the system actually switches the headlamps on, yes — it brings up the full set including the rear position lamps and the registration plate lamp. The problem is the in-between state. In poor daylight the system often does nothing at all, leaving the car on daytime running lamps, which are front-only. So a car in AUTO in heavy rain can quite easily be showing nothing whatsoever at the back.
Is there an MOT check for automatic headlights?: No. Automatic headlamp control is not a testable item, there is no defect for its warning lamp, and no car is required to have it. The headlamps themselves are tested exhaustively for condition, colour, operation, aim and switching, but the tester operates them from the switch. A car whose light sensor has failed will still pass the MOT provided the headlamps work normally when switched on by hand.

This is general guidance, not a diagnosis. Manufacturers draw the same system differently and reuse symbols for more than one function, so check the handbook for your own car before acting on anything here.

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