How do I replace a blown bulb on a car?

Lighting and signalling is consistently the most common MOT failure category in the UK, and it is also the easiest to prevent. The awkward part on many modern cars is not the bulb but reaching it.

Safety

What you'll need

• The correct replacement bulbs
• A clean cloth or gloves
• A torch (optional)
• A small screwdriver or trim tool (optional)

Step by step

1. Check every light on the car first.Sidelights, dipped beam, main beam, front and rear fogs, indicators front and rear and on the mirrors, brake lights including the high-level one, reversing lights and the number plate lights.
2. Check the brake lights on your own.Reverse gently towards a shop window, a garage door or a wall at dusk and watch the reflection, or press the pedal by hand from outside with the door open on many cars. A phone propped up recording behind the car works too.
3. Look up the exact bulb type.The handbook lists them, usually as a code such as H7, W5W or P21W. Buying the wrong one is the most common reason the job stalls halfway.
4. Buy in pairs for headlights.Bulbs dim as they age and fail at similar times. Replacing both keeps the beam even, which matters for the MOT and for seeing.
5. Work out the access route.Most bulbs are reached from behind the unit, through the engine bay or an access hatch. Some cars need the wheel turned and an arch liner flap opened. If it needs the bumper off, hand it to a garage.
6. Photograph everything before disconnecting.Connector orientation, which way the holder was clocked, which screws came from where. Two minutes here saves twenty later.
7. Twist the holder anticlockwise and withdraw it.Most bayonet holders release with a quarter turn. Some headlight bulbs are held by a sprung wire clip that unhooks sideways rather than lifting.
8. Handle the new bulb by its base only.For halogens, use a clean cloth or gloves. Fingerprints create a hot spot on the quartz glass and the bulb fails within weeks. If you touch it, wipe it with a little alcohol.
9. Fit and seat it, then test before reassembling.Refit the connector, switch on and check before you put the covers and clips back. Discovering a bad connection after everything is reassembled is a needless second job.
10. Refit weather seals properly.The rubber boot at the back of a headlight keeps water out. Leaving it half on results in a misted unit and a failed bulb a month later.
11. Check the beam is still aimed correctly.If the light unit was moved or disturbed, the aim needs checking. Badly aimed headlights dazzle others and fail the MOT.

Tips

• Keep a spare bulb kit in the car. Some European countries expect you to carry one and it turns a roadside problem into a ten-minute stop.
• A bulb that blows repeatedly usually means water in the unit or a poor earth, not bad bulbs.
• Number plate lights are the ones nobody checks and are a straightforward MOT failure.

Common mistakes

• Touching the glass of a halogen bulb — The oil from a fingerprint creates a hot spot on the quartz envelope that causes the bulb to fail early, sometimes within days.
• Fitting brighter or non-approved bulbs — Bulbs that exceed the approved rating, or LED replacements in a housing designed for halogen, scatter light, dazzle other drivers and are not road legal in the UK.

If it doesn't work

Questions people ask