Sidelight warning light

What turns it on

Turning the light switch to its first position. The same switch position also lights the rear position lamps and the rear registration plate lamp, which by regulation must come on at the same time.

What to do about it

1. Treat sidelights as a visibility aid, not a driving light. If you can see the tell-tale but the road ahead is dark, you want dipped beam.
2. Use them when parking at night on a road with a speed limit above 30 mph, where leaving a vehicle unlit is not permitted.
3. Check the rear plate lamp occasionally, because it comes on with the sidelights and a car can pass years with a dead one that nobody has noticed. An unreadable rear plate at night is both an MOT defect and an offence.
4. Walk round the car with the sidelights on once in a while. Four small bulbs plus the plate lamp is exactly the sort of thing that quietly fails one at a time.

Can you keep driving?

Yes

Yes, though rarely on sidelights alone. They exist so other people can see you, not so you can see. Once it is dark enough for the tell-tale to matter, dipped beam is what you should be on.

What causes it

• The light switch is in the sidelight position
• An automatic headlamp system has switched to sidelights at dusk before raising dipped beam
• On some cars the daytime running lamps double as front position lamps and dim rather than switch off when the sidelights come on

Does it fail an MOT?

Not on its own — but what causes it often does.

The sidelight tell-tale is not one of the mandatory tell-tales, so it is not itself a defect. The lamps are firmly in the test though: section 4.2 requires two front and two rear position lamps, present, secure, the correct colour and working, and a position lamp that is missing or inoperative is a major defect. The tester also checks they all light from a single operation of the switch and that the registration plate lamp comes on at the same time, so a dead plate lamp is picked up here.

Common questions

Can I drive on sidelights only in the UK?

Almost never in practice. The Highway Code requires headlights at night on any road without lit street lighting, and requires headlights whenever visibility is seriously reduced. That leaves only lit roads, generally those with a 30 mph limit, where sidelights alone would technically satisfy the letter of the rule. Even there it is a poor idea: sidelights put no light on the road, and a car showing only sidelights is much harder to judge for speed and distance than one on dipped beam. Every driving instructor in the country will tell you to use dipped beam.

Do I have to leave sidelights on when parked at night?

It depends on the road. You must not leave a vehicle parked on a road at night without lights unless it is on a road with a speed limit of 30 mph or less, parked close to and parallel with the kerb facing the direction of the traffic flow, and not within ten metres of a junction. On roads with a higher speed limit, sidelights must be left on. Many cars have a parking light function that lights only the kerbside lamps to save the battery.

Do sidelights fail an MOT?

Yes. Under section 4.2 of the inspection manual a car must have two working front and two working rear position lamps, and one that is missing or inoperative is a major defect. A defective lens is a major defect, a lamp that is not securely attached is a minor defect, and one likely to fall off is a major. The tester also checks that the position lamps, the end-outline marker lamps if fitted and the rear registration plate lamp all illuminate together from one operation of the switch.

What is the difference between sidelights and daytime running lights?

Sidelights are front and rear, dim, and intended for darkness. Daytime running lamps are front only, much brighter, and intended to make a car conspicuous in daylight. They are not interchangeable, and confusing them is genuinely dangerous, because a car running on DRLs in poor daytime visibility has nothing lit at the back at all. Some cars use the same lamp units for both, dimming the DRL down to position lamp brightness when the sidelights are switched on, which the MOT manual explicitly permits.

Lights people confuse with this one

• Dipped beam warning light
• Daytime running light indicator
• Bulb failure warning light
• Automatic headlight light

Related reading

• Registration plate missing, insecure or illegible
• Lamp of the wrong colour, position or intensity
• What Is Checked on an MOT? The Complete 2026 List
• The Most Common MOT Failures and How to Avoid Them
• All dashboard warning lights →