What does the blue light on my dashboard mean?

It means main beam is on. Blue is reserved almost exclusively for this one purpose under ISO 2575, the international standard that governs dashboard symbols, precisely so it cannot be confused with anything else. It is not a warning and nothing is wrong. The symbol is a headlamp outline with five straight horizontal lines coming out of it, pointing left; the green version of the same symbol with the lines angled downwards is dipped beam.

Can a car fail its MOT on the main beam warning light?

Yes, and this surprises people. Under section 4.9.1 of the MOT inspection manual the tester must check the mandatory lighting tell-tales, and a tell-tale for main beam or rear fog that is missing or inoperative is a major defect. So a car whose blue main beam lamp no longer lights when main beam is selected fails, even though the headlamps themselves work perfectly. It is genuinely rare: across the DVSA data behind this site the four reason-for-rejection codes covering this defect account for 102 failures in total, which makes it one of the least common failures on the whole site. The rule only applies to vehicles first used on or after 1 April 1986.

When can I use main beam headlights in the UK?

On unlit roads where nobody else will be dazzled. You must dip for oncoming traffic, and you must also dip when you come up behind another vehicle, because main beam in a driver's mirrors is every bit as blinding as it is through a windscreen. The Highway Code prohibits using any lights in a way that dazzles or causes discomfort to other road users, which includes pedestrians, cyclists and horse riders. In fog, heavy rain or falling snow, main beam reflects off the water in the air and reduces what you can see, so dipped beam is both safer and more effective.

Why does my main beam light stay on when I dip the headlights?

That points at the stalk rather than the bulb. The indicator and lighting stalk contains the main beam switch, and the contacts inside it wear, stick or corrode, which can leave the main beam circuit closed even after you have pulled the stalk back to the dipped position. Assume you are actually running main beam and dazzling people until it is checked, because the tell-tale is usually reporting the real state of the circuit. Replacing a stalk switch is not a big job on most cars.

Dashboard warning light

Main beam warning light

Last updated 27 July 2026

Quick answer

Main beam is on. Nothing is wrong: this is the only lamp on a UK dashboard that is blue, and it exists purely so you cannot forget you are dazzling everyone in front of you. Dip it as soon as you meet or catch up with another road user.

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

Why it is blue

Almost exclusively main beam, and a prompt to dip for oncoming traffic.

What turns it on

Selecting main beam with the stalk, or holding the stalk in the flash position, lights the tell-tale. On cars with automatic main beam the same lamp comes on when the system decides to raise the beam, usually alongside a second symbol showing the automatic mode is armed.

What to do about it

Dip as soon as you see oncoming traffic, and dip when you catch up with a vehicle in front — main beam in someone's door mirrors is as blinding as it is head-on.
Dip on lit roads and in built-up areas. Main beam is for unlit roads where you genuinely need the distance.
Dip in fog, rain and falling snow. Main beam reflects straight back off the water in the air and makes visibility worse, which is why dipped beam plus fog lamps is the combination that works.
If the blue lamp does not come on at all when you select main beam, get it fixed before the MOT. The tell-tale is itself a testable item and a dead one is a major defect.
If the blue lamp stays on with the stalk in the dipped position, the stalk or its switch is faulty and you are almost certainly dazzling people without knowing it.

Can you keep driving?

Yes

Completely. A blue main beam lamp is a confirmation, not a warning. The only thing to act on is other people: dip for oncoming traffic, for vehicles you are following, and on lit roads.

What causes it

Main beam has been selected at the stalk, which is the answer nine times out of ten
The stalk is being held or has stuck in the flash-to-pass position
Automatic main beam has raised the beam by itself because it sees no other lights ahead
A worn or sticking indicator stalk assembly holding the main beam circuit closed

Does it fail an MOT?

Yes. A tester records it as a major defect, under the wording A mandatory tell-tale for main beam headlamp or rear fog lamp missing or inoperative in the MOT inspection manual (section 4.9.1).

A main beam tell-tale is only required on vehicles first used on or after 1 April 1986. A rear fog tell-tale is only required where a rear fog lamp is itself mandatory, which covers vehicles first used on or after 1 April 1980.

It is not a technicality either. Across the DVSA test data behind this site, 102 MOTs were failed on it, spread across 38 different models.

Common questions

What does the blue light on my dashboard mean?: It means main beam is on. Blue is reserved almost exclusively for this one purpose under ISO 2575, the international standard that governs dashboard symbols, precisely so it cannot be confused with anything else. It is not a warning and nothing is wrong. The symbol is a headlamp outline with five straight horizontal lines coming out of it, pointing left; the green version of the same symbol with the lines angled downwards is dipped beam.
Can a car fail its MOT on the main beam warning light?: Yes, and this surprises people. Under section 4.9.1 of the MOT inspection manual the tester must check the mandatory lighting tell-tales, and a tell-tale for main beam or rear fog that is missing or inoperative is a major defect. So a car whose blue main beam lamp no longer lights when main beam is selected fails, even though the headlamps themselves work perfectly. It is genuinely rare: across the DVSA data behind this site the four reason-for-rejection codes covering this defect account for 102 failures in total, which makes it one of the least common failures on the whole site. The rule only applies to vehicles first used on or after 1 April 1986.
When can I use main beam headlights in the UK?: On unlit roads where nobody else will be dazzled. You must dip for oncoming traffic, and you must also dip when you come up behind another vehicle, because main beam in a driver's mirrors is every bit as blinding as it is through a windscreen. The Highway Code prohibits using any lights in a way that dazzles or causes discomfort to other road users, which includes pedestrians, cyclists and horse riders. In fog, heavy rain or falling snow, main beam reflects off the water in the air and reduces what you can see, so dipped beam is both safer and more effective.
Why does my main beam light stay on when I dip the headlights?: That points at the stalk rather than the bulb. The indicator and lighting stalk contains the main beam switch, and the contacts inside it wear, stick or corrode, which can leave the main beam circuit closed even after you have pulled the stalk back to the dipped position. Assume you are actually running main beam and dazzling people until it is checked, because the tell-tale is usually reporting the real state of the circuit. Replacing a stalk switch is not a big job on most cars.

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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