Number plates, lights, and horn: three categories that between them account for a significant slice of avoidable MOT failures every year. None require mechanical knowledge to check. Most failures cost under £20 to fix if caught in advance. Found by the tester on the day, they add to the failure total and push you into a second appointment.
This check takes five minutes. You need someone to sit in the car and operate things on request, a torch, and daylight.
Number plates
UK number plate rules are more specific than most drivers realise. The failure rate for plate non-conformance is low but disproportionate given how easy it is to check.
The required font is Charles Wright 2001, a stroke-width and letter-shape defined in British Standard BS AU 145d. Every digit and letter in that font is specified — character height (79mm), width (50mm, or 44mm for the numeral 1), character stroke width (14mm), and the spacing between characters. These figures apply to both standard and motorbike plates.
If your plate uses a different font — particularly the italic-style fonts sold on novelty plates and through some online plate suppliers — it does not conform. A tester assessing plate legibility under RFR 10.3.1 will note the non-conforming font as a fail. The same applies to plates with 3D-effect letters that alter the character stroke, or plates where the text has been stylised (characters condensed or spaced unevenly to make the registration fit an unusual format).
Check the plate itself:
- Font: characters should be upright, standard-width. No italics, no compressed fonts, no gel-raised letters that create a shadow effect from the front.
- Spacing: there should be a fixed gap between the area code, the year identifier, and the random letters — no custom spacing to make the registration spell something at a distance.
- Condition: cracks, missing letters (letterbox theft is still common), or damage that obscures part of the registration is a fail. The plate needs to be readable.
- Reflectivity: the front plate should be white reflective, the rear plate yellow reflective. A faded or yellowed front plate or a front plate mounted in a recessed area with an aftermarket tinted cover over it — all fail.
- Fixings: the plate needs to be secure. A plate held on by one screw and vibrating is a fail risk under the vehicle condition category.
Lights — the full circuit
Walk around the car with the ignition on and a helper operating each circuit in turn. Work through the following systematically:
Side (position) lights. Small bulbs in the front and rear lamp cluster. Switch them on — they should illuminate at both front corners and both rear corners. Often combined with the tail lamp in some vehicles, so confirm they operate independently on the sidelights switch.
Dipped headlamps. Both should operate. The beam should be clean — no internal condensation in the headlamp unit causing a diffuse, scattered output. Condensation inside a sealed headlamp unit is not itself an automatic fail at every test, but significant condensation that obscures the bulb and degrades the beam can be assessed under lamp function. It’s worth noting.
Main (full) beam. Both should operate and the main beam indicator on the dashboard should illuminate.
Fog lamps. Front fog lamps, where fitted, should operate. Rear fog lamps are required on all cars manufactured after a certain period and must operate. Both lamps should light — a single working rear fog lamp where two were originally fitted is assessed against the original specification.
Brake lights (stop lamps). Have your helper hold the brake pedal while you check all stop lamps. Front-mounted third brake lights (high-level stop lamps) are required on cars type-approved after October 2001 — check the lamp in the rear screen or spoiler. On cars type-approved before that date, a third light may be fitted but is not required, though if it’s fitted and doesn’t work, that’s a fail.
Reversing lights. Have your helper select reverse with the handbrake applied. One or two reversing lamps should illuminate. If no reversing lamp was originally fitted (common on older vehicles), absence isn’t a fail. If one was fitted and doesn’t work, it fails.
Indicators. Press the hazard switch. Walk around and confirm all four corners blink: two fronts, two rears, plus side repeaters where fitted. The flash rate should be roughly 1–2 flashes per second. A flash rate that’s noticeably faster than this (“hyperflashing”) usually means one bulb in that circuit has failed — the remaining bulbs try to compensate by drawing more current. Finding the dead bulb and replacing it fixes the flash rate.
Number-plate lights. These sit above or around the rear plate and are required to illuminate when the side or dipped lights are on. They’re easily missed and produce an embarrassing Major defect for a bulb that costs about £2. Shine a torch at the plate area and confirm both lamps are lit.
Cracked indicator lenses and water ingress
This is the specific failure most commonly overlooked in the lights check. A cracked indicator lens — even a hairline crack — allows water to enter the lamp unit. Water inside an indicator is a direct problem for two reasons: it corrodes the bulb contact and accelerates bulb failure, and it obscures the lens. A water-filled indicator that looks orange but produces only a dim, refracted glow through the pooled water fails the function check.
On older cars, check every indicator and brake light lens for cracks, cloudiness, or visible moisture. Replacement indicator lenses are available for most common models for £10–£25. Where the full lamp unit is needed, £25–£60 covers most hatchbacks. Fitting typically takes under thirty minutes.
LED lamp units on newer cars rarely suffer this problem because they’re sealed and generate less heat. On cars over ten years old with incandescent bulbs, cracked lenses are a regular occurrence.
Horn
The horn check is one of the simplest items on the test: press the horn, confirm it produces a sound audible from outside the vehicle, confirm the sound comes from the front of the vehicle.
A horn that works when you press it in the car but can’t be heard clearly at three or four metres distance from outside — because the horn is pointing backward into the engine bay, because the horn grille is blocked, or because the horn itself is deteriorating — fails under RFR 7.7.1. The requirement is that the horn can be heard “from outside the vehicle.” Audible from inside while the windows are up is not the test.
The most common horn failure on older vehicles is a corroded horn earth connection or a corroded connector at the horn unit itself. The horn may work intermittently, or may sound weak. A replacement horn unit costs £10–£20 and the fitting is straightforward on most cars — one mounting bolt and two electrical connectors.
On vehicles where the horn is activated through the steering wheel airbag clockspring, a failed horn is sometimes a sign that the clockspring is wearing. This is a more involved repair, but it’s better to know before the test than after.
Headlamp condensation and LED failure modes
Two issues worth mentioning separately because they affect newer cars differently from older ones.
Internal condensation in a headlamp unit is common on cars over five years old where the moisture seal in the lamp housing has degraded. A small amount of temporary condensation that clears when the lamp warms up is usually not assessed as a fail. A significant pool of moisture sitting inside the lens that visibly clouds the beam output is a different matter — the beam efficiency drops, and a tester can fail the lamp on function rather than just recording it as an advisory.
If your headlamp has persistent heavy condensation, the fix is usually a replacement lamp unit or a resealing job. Some motor factors carry resealing kits; a specialist can inject a fresh seal. If the condensation keeps returning, the housing has cracked somewhere and needs replacing.
LED lamp failures on newer cars present differently from bulb failures. A failed single LED in a multi-LED array sometimes dims the lamp noticeably but doesn’t extinguish it entirely. The driver may not notice. A tester assessing lamp brightness output can flag an LED array that’s operating at reduced intensity as a defect even without a full lamp-out. If any part of your LED lamp looks darker or has dead sections in the array, the unit needs attention.
Combining this with the full walkaround
The lamp check here overlaps with Check 1 in the pre-MOT walkaround, which covers the same circuit more briefly as part of a six-point routine. This guide goes into more depth on plate compliance, lens condition, and horn specifics. If you’re working through both, combine the lamp circuit check into one circuit rather than doing it twice.
The complete light check takes about five minutes with a helper. Number plates take another two if you know what you’re looking for. Horn test takes thirty seconds. The total investment is under ten minutes, and the failure rate for these items at UK MOT stations is high enough that this is not a theoretical exercise.
For an estimate of what bulb replacements and minor electrical repairs cost across different centre types in your area, the MOT cost estimator gives a current regional range. Number-plate replacement is straightforward to price independently — most plate suppliers offer an online preview that shows the correct Charles Wright font before you order. Don’t order a plate you can’t legally use.
If you’re curious how often these items appear on UK MOT failure records, the failures index at /failures/ breaks down defect rates by RFR code. Lighting defects appear in volume across every vehicle class, every year.