How to change a car bulb

Safety: only do jobs you are confident with. Never work under a car supported only by a jack, and if a repair affects brakes, steering or suspension and you are unsure, have a garage do it or check your work.

Lights and electrics are the most common reason cars fail their MOT. In 2025, 16.4% of all car MOT tests failed on something in this area, and a blown bulb is the usual culprit. It is also the cheapest failure to avoid: most bulbs cost a few pounds and take minutes to change.

Before you start

• Find the right bulb. Your handbook lists the type for each light (for example H7 for many headlights, P21W for brake lights, W5W for number-plate lights). Take the old bulb to a motor factor if you are unsure.
• Turn the lights and ignition off, and let a headlight bulb cool for a few minutes. Halogen bulbs run hot enough to burn.
• Don’t touch the glass of a halogen bulb. Oil from your skin creates a hot spot that shortens its life. Hold it by the metal base, or use a clean cloth.
• Leave xenon (HID) and built-in LED units to a garage. Xenon headlights run at very high voltage, and many LED lamps can only be replaced as a whole unit.

Headlight bulbs

1. Open the bonnet and find the back of the headlight unit. On many cars there is a round or square rubber cover over each bulb.
2. Pull off the cover. Unplug the wiring connector from the bulb by squeezing its tab.
3. Release the bulb. Most are held by a wire spring clip (press it in and swing it aside) or a twist-lock base (turn anticlockwise about a quarter turn).
4. Fit the new bulb the same way round; the base only fits one way. Refit the clip, the connector and the rubber cover. A missing cover lets water in and causes condensation.
5. Test dipped and main beam before closing the bonnet.

On some cars there is no room behind the headlight and you go in through the wheel-arch liner, or the bumper has to come off. Your handbook will say. If the bumper needs removing, it is reasonable to let a garage do it.

Brake lights, indicators and rear lights

1. Open the boot. Look for a small cover or removable panel in the trim behind the light cluster.
2. The bulbs sit in a plastic holder. Twist the holder anticlockwise and pull it out, or on some cars undo two or three screws and lift the whole cluster away.
3. Push the old bulb in gently and twist it anticlockwise to release it. Many rear bulbs have offset pins and only fit one way.
4. Fit the new bulb, refit the holder, and check it works. To check brake lights alone, reverse up to a garage door or window at night and look in the mirror, or wedge the pedal down with a piece of wood.

If an indicator flashes much faster than normal, a bulb on that side has failed. Indicators must show amber; a bulb whose coating has worn off and now shows white is an MOT failure.

Number-plate lights

These are easy to forget and an MOT failure if either is out. Most clip in: push the lens towards one end with a small flat screwdriver wrapped in tape, and it pops out. The small push-fit bulb (often W5W or a festoon bulb) pulls straight out. If the lens is cracked or full of water, a replacement lens costs a few pounds.

If the new bulb doesn’t work

• Check the fuse for that light, using the fuse chart on the box lid or in the handbook.
• Look for green corrosion in the bulb holder. Clean it with a small wire brush or fine sandpaper.
• If one side works and the other doesn’t with a known-good bulb, the fault is in the wiring or earth, which is a job for an auto electrician.

After changing a headlight bulb

Headlight aim is checked at the MOT, and a badly seated bulb can throw the beam out. Park facing a wall at night: the dipped beam should have a flat top with a step up to the left, and both sides should be at the same height. If one looks noticeably higher or lower, check the bulb is seated properly before adjusting anything.

Which cars fail on lights and electrics most?

From our analysis of every car MOT in 2025, these popular models failed on lights and electrics most often. If you own or are buying one, it is worth checking this area more carefully. Older cars naturally fail more, so the figures partly reflect each model’s age.

See every model in our MOT data by model.

Lighting checks before an MOT

• Dipped and main beam, both sides
• Side lights and rear lights
• Brake lights, including the high-level one
• All four indicators and the hazard lights
• Rear fog light and its dashboard warning
• Both number-plate lights
• Reversing light

See our full MOT preparation checklist for everything else to check.