Why is my bulb failure light on when all my lights work?

Three things account for most of these. The first is that you have not found the dead lamp yet — the brake lamps, the high-level brake lamp and the rear registration plate lamp are invisible from the driver's seat and are the usual culprits. The second is LED bulbs fitted in place of filament ones: they draw far less current, so the monitoring circuit reads them as failed even though they are clearly working, and the fix is a load resistor or a CAN-bus compatible bulb. The third is a corroded holder or poor earth adding enough resistance to trip the monitoring while the lamp still lights.

Will a blown bulb fail an MOT?

Yes, and it is one of the most common failures there is. A headlamp, position lamp, stop lamp, indicator or obligatory fog lamp that does not work is a major defect, which means a straight fail. An unlit rear registration plate lamp is a defect too. It is a particularly annoying way to fail because bulbs are cheap, most take minutes to change, and a two-minute walk round the car with someone pressing the pedal and working the switches would have caught it the week before.

Can the police stop me for a blown bulb?

Yes. Driving with a defective obligatory lamp is an offence under the lighting regulations, and it is one of the most common reasons for a routine stop. In many cases the outcome is a vehicle defect rectification notice, which requires you to have the fault repaired and the repair certified within a set period, but a fixed penalty is also possible. A dead brake lamp or indicator is treated more seriously than a dim plate lamp, for obvious reasons.

Should I change headlight bulbs in pairs?

For filament bulbs, yes. Halogen bulbs dim gradually throughout their life and both sides have done the same hours, so when one fails the other is usually close behind and already noticeably dimmer. An uneven pair also produces a lopsided beam pattern that makes judging the road edge harder at night. LED and HID units are different — those are expensive enough that most people replace only the failed side, and their output falls off far more slowly.

Dashboard warning light

Bulb failure warning light

Last updated 27 July 2026

Quick answer

The car has detected that one of its exterior lamps is not drawing the current it should, which usually means a bulb has blown. It is a genuine fault warning rather than a confirmation lamp, and since a lamp not working is among the most common reasons cars fail the MOT, it is worth acting on straight away.

What to do: Get it checked within days
Can you drive?: Yes

Why it is amber

A system has reported a malfunction, or something needs attention. The car is usually still driveable, but the fault will not clear itself and several amber lamps are an automatic MOT failure.

What turns it on

The body control module measures the current through each monitored lamp circuit. An open circuit, or a draw well outside the expected range, is read as a failed lamp and lights the warning. Some cars name the offending lamp on the driver's display; others give you nothing but the symbol and leave you to find it.

What to do about it

Find the dead lamp before assuming which one it is. Get someone to work the switches while you walk round: dipped beam, main beam, sidelights, indicators both sides, fog lamps, reverse, and the rear registration plate lamp.
Do not forget the brake lamps and the high-level brake lamp. They are the ones you can never see from the driver's seat and they are a very common cause of this warning.
Replace filament bulbs in pairs where they work as a pair, such as headlamps and brake lamps. They age at the same rate and the second one rarely lasts long after the first.
If the lamp came on straight after fitting LED replacement bulbs, that is the cause. LEDs draw a fraction of the current of the filament bulbs the circuit expects, so the car reports them as failed even though they are lit.
If everything visibly works and the warning stays on, suspect a poor earth or a corroded bulb holder rather than the bulb. A high-resistance connection changes the current enough to trip the monitoring even while the lamp still lights.

Can you keep driving?

Yes

The car drives normally, but you may be driving without a brake lamp or an indicator and not know it. That is an offence on the road as well as a major MOT defect, and police forces do stop cars for it. Bulbs are among the cheapest parts on any car and most are a five-minute job, so there is no good reason to leave this one.

What causes it

A blown filament bulb, most often a brake lamp, an indicator or the rear registration plate lamp
LED bulbs fitted to a circuit designed to monitor filament bulbs
A corroded bulb holder or a poor earth in a rear lamp cluster, which is endemic on older cars
Water in a lamp unit from a perished seal or a cracked lens
A failed LED segment inside a sealed rear lamp cluster, which usually means replacing the whole unit
A bulb of the wrong wattage fitted at some point in the car's past

Does it fail an MOT?

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

The bulb failure warning lamp is not itself a listed defect — it is not one of the four mandatory tell-tales. What it is telling you about almost always is. Lamps that do not work are among the most common MOT failures of all: an inoperative headlamp under 4.1, position lamp under 4.2, stop lamp under 4.3, indicator under 4.4 or fog lamp under 4.5 is a major defect in each case, and an unlit rear registration plate lamp under 4.7 is a defect too. Direction indicator lamp defects alone account for 105,791 failures in the DVSA data behind this site.

Common questions

Why is my bulb failure light on when all my lights work?: Three things account for most of these. The first is that you have not found the dead lamp yet — the brake lamps, the high-level brake lamp and the rear registration plate lamp are invisible from the driver's seat and are the usual culprits. The second is LED bulbs fitted in place of filament ones: they draw far less current, so the monitoring circuit reads them as failed even though they are clearly working, and the fix is a load resistor or a CAN-bus compatible bulb. The third is a corroded holder or poor earth adding enough resistance to trip the monitoring while the lamp still lights.
Will a blown bulb fail an MOT?: Yes, and it is one of the most common failures there is. A headlamp, position lamp, stop lamp, indicator or obligatory fog lamp that does not work is a major defect, which means a straight fail. An unlit rear registration plate lamp is a defect too. It is a particularly annoying way to fail because bulbs are cheap, most take minutes to change, and a two-minute walk round the car with someone pressing the pedal and working the switches would have caught it the week before.
Can the police stop me for a blown bulb?: Yes. Driving with a defective obligatory lamp is an offence under the lighting regulations, and it is one of the most common reasons for a routine stop. In many cases the outcome is a vehicle defect rectification notice, which requires you to have the fault repaired and the repair certified within a set period, but a fixed penalty is also possible. A dead brake lamp or indicator is treated more seriously than a dim plate lamp, for obvious reasons.
Should I change headlight bulbs in pairs?: For filament bulbs, yes. Halogen bulbs dim gradually throughout their life and both sides have done the same hours, so when one fails the other is usually close behind and already noticeably dimmer. An uneven pair also produces a lopsided beam pattern that makes judging the road edge harder at night. LED and HID units are different — those are expensive enough that most people replace only the failed side, and their output falls off far more slowly.

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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