What a Licence Plate Light Does
A licence plate light illuminates the rear registration plate so that it can be read from 20 metres at night. The lamp shines white onto the surface of the plate, not directly toward following drivers, and reflects off the plate’s retroreflective coating. The combined effect makes the registration number legible to following vehicles, police automatic number plate recognition (ANPR) cameras, and parking enforcement systems after sunset.
The licence plate light goes by several names in the UK: number plate light, number plate lamp, plate light, registration plate lamp. Each name refers to the same component. The term “licence plate light” is more common in product listings and US-influenced workshop literature. The term “number plate light” is more common in UK MOT and DVSA documentation. Both terms mean the white lamp mounted above or beside the rear plate.
A licence plate light is a passive component. It does not flash, it does not change colour, and it does not signal to other road users. It exists purely to keep the registration plate readable. The lamp wires to the same circuit as the rear sidelights, so it comes on whenever the operator switches on dipped beam, main beam, or parking lights.
UK Legal Requirements for Number Plate Lights
UK law requires every motor vehicle to display a working white licence plate light at the rear from sunset to sunrise. The Road Vehicles Lighting Regulations 1989 set the requirement under Schedule 2. The Vehicle Excise and Registration Act 1994 adds the requirement to keep the registration plate legible at all times the vehicle is on a public road.
Three rules apply. The light must show white only (never amber, blue, or coloured). The light must illuminate the plate sufficiently for the number to be read from 20 metres. The light must come on with the rear sidelights and stay on throughout dipped and main beam use.
A car or van without a working number plate light is illegal to use on a public road in darkness. The same rule applies to tractors when used on public roads, to agricultural trailers carrying a rear plate, and to lorries and HGVs. The fine for driving with a non-working number plate light at night is £100 fixed penalty and three points on the driving licence.
The same rule does not apply to front number plates. A front plate must be displayed and must be legible in daylight, but UK law does not require a light to illuminate the front plate. Only the rear plate needs illumination.
Bulb Types and Fitment
Most licence plate lights use a C5W festoon, W5W capless, or T10 wedge bulb. The three bulb types fit different lamp housings and the wrong replacement will not seat in the holder.
A C5W festoon bulb is a 5W incandescent bulb with two spring-loaded metal end caps. The bulb measures 38mm or 41mm in length. The lamp holder grips both end caps and the bulb sits horizontally across the housing. C5W festoons are common on older European cars and on many UK agricultural trailers built before 2010.
A W5W capless bulb is a 5W incandescent bulb with a glass-stem base that pushes into a plastic socket. The bulb measures 27mm in overall length. W5W bulbs appear in most modern car number plate lamps, in trailer lamps from 2010 onwards, and in many universal lamp bodies.
A T10 wedge bulb is the same physical fitment as a W5W but the term covers both filament and LED variants. T10 LED replacements use the same wedge base as a W5W filament bulb but draw 0.5W to 1W and emit cool white light from a cluster of surface-mount LEDs. The vehicle bulb types article covers the full bulb classification system.
A small number of older vehicles use BA9s or BA15s bayonet bulbs in the plate lamp. These pre-date the move to capless and festoon types and are now found mainly on classic cars and on heritage tractors built before 1980.
Replacing a Licence Plate Bulb
Replace a licence plate bulb by removing the lens cover, pulling out the failed bulb, and pushing in a matching replacement. The job takes two to five minutes on most vehicles and needs only a small Phillips screwdriver or a flat trim tool.
Step one: locate the lamp housing. The licence plate lamp sits above or beside the rear plate. Two small Phillips screws or two plastic clips hold the lens cover in place. Some lamps recess into the bumper or boot lid trim and need a trim removal tool to release the clips.
Step two: remove the lens cover. Unscrew or unclip the cover and lift it away from the housing. Behind the cover sits the bulb in its holder. A C5W festoon sits in two spring-loaded clips. A W5W capless sits in a friction-grip socket.
Step three: remove the failed bulb. Pull the C5W festoon straight out from one clip and slide it sideways out of the other. Pull the W5W capless straight out of its socket. Check the contacts for corrosion or staining. Clean any green deposit with a cotton bud dipped in contact cleaner.
Step four: fit the replacement. Push the new bulb into the holder in the same orientation as the old one. Test the light by switching on the rear sidelights. Refit the lens cover and tighten the screws or push home the clips.
LED Number Plate Light Upgrades
LED licence plate bulbs replace filament bulbs with 0.5W to 1W LEDs that last 30,000 hours. The upgrade swaps a 5W incandescent bulb for an LED cluster in the same physical base. The lamp housing stays the same, the wiring stays the same, and the lens stays the same.
The visible difference is the colour and the brightness. A filament licence plate bulb produces 2,700K to 3,000K warm white light. An LED replacement produces 5,000K to 6,500K cool white light. The cooler colour reflects more brightly off the retroreflective plate coating and makes the number legible from a longer distance. The ANPR camera also reads the plate more reliably under LED illumination.
The hidden difference is the power draw. A filament 5W bulb draws 0.4A at 12V. An LED 0.5W bulb draws 0.04A. A pair of LED plate bulbs cuts the sidelight circuit load by 0.7A, which matters on older tractors with marginal alternators or on trailers running long umbilical cables.
LED licence plate bulbs also survive vibration better than filament bulbs. The LED has no fragile tungsten filament to fracture under the constant shake of a working agricultural trailer or the bumps of a country lane. Most LED plate bulbs carry a CANbus-friendly resistor or capacitor inside the base to prevent the car’s lighting control module from registering a bulb failure warning.
Are LED Number Plate Lights Legal in the UK
LED licence plate lights are road legal in the UK provided they show white light and carry an E-mark. The E-mark certifies the bulb meets the ECE Regulation 37 standard for vehicle bulbs or the ECE Regulation 128 standard for LED light sources.
An LED plate bulb without an E-mark is not road legal. The lamp housing remains legal because it was approved when the vehicle was type-approved, but the bulb inside it must carry its own E-mark to meet the substitution rules. Cheap unbranded LED plate bulbs sold without certification can lead to an MOT advisory note or a roadside fine.
Three additional rules apply. The LED must show white only (no blue tint, no purple cast, no coloured edge effect). The LED must illuminate the plate evenly (no dark patches between LED dots on the plate surface). The LED must not project light backward toward following drivers (the housing must shield the LED from direct rear view).
Most reputable LED plate bulb brands meet all three rules and carry both the E-mark and a CE conformity mark. Examples include Osram LEDriving, Philips X-tremeUltinon LED, and Ring Automotive’s Premium LED range. Cheap eBay imports often fail one or more rules and have no E-mark stamped on the base.
Number Plate Lights and the MOT Test
An MOT tester fails a vehicle when a licence plate light fails to work or the lens is cracked or missing. The MOT inspection manual Section 4.7 covers number plate lamps and lists three failure conditions.
Failure condition one: the lamp does not illuminate when the sidelights are switched on. The tester checks the lamp by switching on the rear lights and observing whether the plate is illuminated. A blown bulb, a broken wire, or a corroded earth all trigger this failure.
Failure condition two: the lens is missing or insecure. The tester checks the lens by visual inspection. A cracked lens is a minor advisory if the lamp still works. A missing lens or a lens that allows direct rear-facing light leakage is a fail.
Failure condition three: the wrong colour shows from the rear. The tester checks the colour by visual inspection. A blue or purple LED, an amber filament, or a damaged red-tinted lens are all failures.
Failure rates for number plate lights run at around 1.2% of all MOT tests according to recent DVSA data. The fault is one of the easiest to fix at the test station: a £3 replacement bulb on the spot retests as a pass. Many testers carry a stock of C5W and W5W bulbs and offer the fix as a chargeable extra.
Fitment on Tractors and Agricultural Trailers
Tractor and trailer licence plate lights mount above or beside the rear plate and run from the sidelight circuit. The lamp housing on a tractor sits on the rear mudguard or on the cab rear pillar. The lamp housing on a trailer sits on the rear panel above the plate or on a small bracket beside it.
Most modern tractors carry a plate lamp at the rear right hip, integrated into the mudguard light cluster. The light shines downward and forward onto the plate face. The same circuit feeds the sidelight, the position lamp, and the plate lamp through a single trailer-compatible loom. The tractor number plate lights article covers tractor-specific fitment in detail.
Agricultural trailers fit a plate lamp on the rear cross-member or on a dedicated bracket above the plate holder. The trailer plug carries the sidelight feed on pin 5 (white) for a 7-pin 12N plug or on pin 1 (yellow) for a 13-pin Euro plug. The plate lamp wires into this feed alongside the rear sidelights.
A retrofit plate light kit for older trailers includes a lamp body, two stainless steel screws, a 200mm wire tail, and a Scotchlok connector. Fitting takes 15 minutes per trailer once the existing wiring run is identified. LED kits cost £8 to £15. Filament kits cost £4 to £8.
Troubleshooting a Licence Plate Light That Does Not Work
A licence plate light that does not work usually has a blown bulb, a corroded earth, or a broken sidelight fuse. Check the three causes in that order: bulb, earth, fuse.
Bulb first. Pull the bulb from the housing and inspect the filament or LED. A blackened filament glass or a snapped tungsten coil means the bulb has blown. Fit a fresh bulb of the same type (C5W, W5W, T10) and test.
Earth second. The plate lamp earth runs through the lamp body to the vehicle chassis or trailer chassis. Corrosion at the contact point blocks the earth and stops the light working even with a fresh bulb. Unbolt the lamp, clean both the lamp body and the chassis face with a wire brush, and refit with a fresh stainless steel bolt or screw.
Fuse third. The sidelight fuse on most cars sits in the engine bay fuse box or the dashboard fuse box rated at 10A. Pull the fuse and inspect the wire bridge inside the plastic body. A broken bridge means the fuse has blown. Replace with a fuse of the same rating (never higher) and test.
If all three checks fail to restore the light, the lamp wiring has an open circuit between the fuse box and the lamp itself. Trace the wire with a multimeter set to continuity mode, working from the bulb holder back toward the fuse box. Common break points include the boot lid hinge, the tailgate harness grommet, and the trailer plug connector.
Choosing the Right Licence Plate Light
Choose a licence plate light by matching the lamp housing to the vehicle and the bulb to the housing. A flush-fit lamp suits modern cars and trailers with a recessed cutout in the rear panel. A surface-mount lamp suits older trailers and tractors without a pre-cut hole. A bracket-mount lamp clips onto the existing plate frame and saves drilling new holes.
The bulb specification follows the lamp housing. Check the housing’s existing bulb (C5W, W5W, or T10) and order a matching replacement. If upgrading to LED, order an LED bulb with the same base type and an E-mark stamped on the body.
Look for the IP rating on the lamp housing. An IP65 rating suits a car or a covered trailer. An IP67 rating suits an open agricultural trailer exposed to slurry and pressure-washing. An IP69K rating suits a beet harvester, a manure spreader, or any vehicle pressure-washed at high temperature.
The full Agri Lighting universal licence plate lamps range lists every approved lamp by mounting type, voltage, and bulb fitment. A working licence plate light is one of the cheapest and easiest road-legal components to fit and one of the most common reasons for an MOT failure or a fixed penalty notice. Keep a spare C5W and a spare W5W in the cab toolbox and the problem rarely lasts more than five minutes.
Internal links pointing to articles not yet published:
- /lighting-regulations/mot-lighting-requirements/ (article 8.6, P2, not yet written) – referenced indirectly in MOT section. Add direct link when published.