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Why lights matter so much at an MOT

Ask any tester what scuppers the most MOTs and they will usually say the same thing: something simple that the driver could have spotted in five minutes on the driveway. Bulbs sit right at the top of that list. A blown brake light or a dull indicator is a minor fault in the grand scheme of things, but the tester has no discretion. The lamp either works, shows the right colour, and can be seen clearly, or the car fails. That means a £3 bulb can cost you a retest appointment, another trip across town, and a day without the car.

The fix is straightforward. About a week before your test, set aside twenty minutes and a helper, and work through every lamp on the car in a logical order. Write down anything that does not light up, replace it, then check it again. If you do that properly, you have removed one of the most common reasons for a fail before the test even begins.

Find a helper and work as a team

Brake lights are the awkward ones. You cannot press the pedal and watch the rear of the car at the same time, and a wall or shop window only gives you a rough idea. A second person makes the whole job quick and certain. Park on a flat, level surface with plenty of room to walk around the car, ideally with the handbrake on and the engine off unless you need the ignition for the lights to work. Many cars need the ignition switched on for the lamps to operate at all, so check yours first.

Sit your helper in the driver's seat. Agree the order before you start so neither of you loses track, and ask them to hold each setting for a good few seconds while you walk around and look at every corner of the car. If you genuinely cannot find a helper, prop your phone against something behind the car and record a short video while you press the brake pedal, then watch it back. It works, though it is slower and easy to misread in daylight.

Work through every lamp, one at a time

Go through this list in order and call out each result so your helper can note it down:

• Sidelights and tail lights — front and rear, both sides, plus the number plate lamps. Number plate bulbs are the classic forgotten failure and take seconds to check.
• Dipped beam — both headlamps, checking that the beam pattern looks even and that neither lamp is noticeably dimmer or yellowed.
• Main beam — including the blue warning light on the dashboard.
• Indicators — front, rear and any repeater on the wing mirror or front wing, on both sides. Watch the speed of the flash: a rapid tick usually means a bulb has gone.
• Hazard lights — all four corners flashing together, with the tell-tale clicking inside.
• Brake lights — including any high-level brake light in the rear window or spoiler, which many drivers never think to check.
• Rear fog lamp — at least one must work on cars first used from April 1980 onwards, and the dashboard warning light should come on with it.
• Reverse lamps — with the ignition on and the car in reverse, engine off and handbrake firmly applied.
• Front fog lamps, if your car has them. They are not part of the test if they are not fitted, but if they are there and working incorrectly, they can attract attention.

Check the colour as well as the function. Indicators must flash amber, brake and rear fog lamps must be red, and reverse lamps must be white. A lens that has faded to almost clear, or a bulb with the wrong tint, is a fail even if it lights up.

Replacing a bulb the right way

Buy the correct bulb for your car rather than the nearest match on the shelf. The handbook, the old bulb itself, or the markings on the holder will tell you the type and wattage. A bulb of the wrong rating can be too bright, draw too much current, or simply not fit properly. If the lamp is a halogen, avoid touching the glass with bare fingers, as the oils from your skin shorten its life dramatically. Hold it in a clean cloth or the packaging.

While the holder is out, look for corrosion, water droplets or a melted connector, all of which point to a deeper problem that a new bulb will not solve. Push the bulb in firmly and squarely, refit the seal and cover properly, and test it before you put all the trim back. If a new bulb fails within a day or two, suspect the holder or the wiring rather than the bulb.

Look beyond the bulbs

Bulbs are only part of the picture. Cracked or badly clouded lenses scatter the light and can fail on beam pattern or visibility, so clean them with a proper headlight restorer or replace the unit if the damage is serious. Headlamp aim is also tested, and it is easily knocked out by a pothole or a recent bump. Many garages will check and adjust aim for a small charge, and it is worth doing before the test rather than after.

Also check for condensation inside the lamps, a sign the seal has failed, and look at any tow bar socket if you have one, as its wiring is tested too. Finally, make sure no dashboard bulb failure warning light is glowing before you set off for the test centre.

The night-before routine

Repeat the whole check the evening before your appointment. It takes ten minutes, and it catches the bulb that was on its way out when you looked last week. Wipe the lenses, top up the screenwash, and make sure the washers and wipers work, since a grimy windscreen can affect the tester's view of the road. Then take the written list of anything you could not fix and mention it when you hand over the keys. Testers appreciate a driver who has clearly done their homework, and a car that arrives with every lamp working starts the day on the right footing.