Number Plate Light Not Working MOT Advisory: Easy Fix
A number plate light advisory is one of the simplest and cheapest MOT fixes, often £2 and 2 minutes. Covering the DIY replacement, LED options, common failure causes, and MOT rules.
Will a Number Plate Light Fail the MOT?
Yes: but it depends on severity. The MOT requires the rear number plate to be clearly illuminated by a white light when headlights or sidelights are on. This is a legal requirement under UK road lighting regulations (Road Vehicles Lighting Regulations 1989).
| Scenario | MOT Result |
|---|---|
| One of two number plate lights not working | Advisory (plate still partially lit) |
| Both number plate lights not working | Fail |
| Light working but very dim | Advisory |
| Light emitting wrong colour (blue, yellow) | Fail |
| Light housing cracked but bulb working | Advisory |
| Light housing missing | Fail |
| Aftermarket LED causing flicker | Advisory or fail depending on visibility |
A single bulb failure on a car with two number plate lights is typically an advisory, the plate is still partially illuminated. But if you have only one light housing (some older cars), a single bulb failure means a fail.
How to Fix It Yourself: Under £5, Under 5 Minutes
This is genuinely one of the easiest car repairs you'll ever do. No tools required on most cars.
What You'll Need
- Replacement bulb: W5W (501 wedge), the standard number plate bulb on 90% of UK cars. Cost: £1 to £3 for a pair from Halfords, any motor factor, or even most petrol stations
- Small flat-head screwdriver (for some light housings)
- Clean cloth (don't touch halogen bulbs with bare fingers)
Step-by-Step Process
- Locate the light housing: On the rear of the car above or beside the number plate. Usually held in by two screws or a press-fit clip
- Remove the housing: Either unscrew the two retaining screws, or press the clip and slide the housing out. Some cars (VW, Ford) have a quarter-turn twist-lock mechanism
- Remove the old bulb: Pull the W5W wedge bulb straight out of its socket. It's a friction fit, no twisting required
- Insert the new bulb: Push the new W5W bulb into the socket until it clicks firmly. Use a clean cloth, finger oils on halogen bulbs cause hot spots
- Refit the housing: Push or screw it back into position. Ensure it sits flush against the bodywork to prevent water ingress
- Test: Turn on your sidelights and check the plate is illuminated from behind the car
Total time: 2 to 5 minutes. Total cost: £1 to £3.
Car-Specific Variations
| Car | Access Method | Bulb Type |
|---|---|---|
| Ford Fiesta / Focus | Press clip, slide out | W5W (501) |
| VW Golf / Polo | Quarter-turn twist-lock | W5W (501) |
| BMW 1/3 Series | Two screws | LED module (see below) |
| Vauxhall Corsa / Astra | Press clip | W5W (501) |
| Toyota Yaris / Corolla | Press clip or screws | W5W (501) |
| Audi A3 / A4 | Press clip, pull down | W5W or LED module |
LED Number Plate Lights: Worth the Upgrade?
LED replacement bulbs are available for most cars at £3 to £10 per pair. They offer:
- Brighter, whiter light: better plate illumination
- Longer lifespan: 20,000 to 50,000 hours vs 1,000 hours for halogen
- Lower power consumption: negligible but contributes to longevity
MOT Compliance for LEDs
LEDs must meet specific requirements to pass the MOT:
| Requirement | Detail |
|---|---|
| Colour | Must emit white light only. No blue tint, no colour shifting |
| Brightness | Must illuminate the plate clearly from 20 metres (standard MOT check distance) |
| Flicker | Must not flicker or strobe. Some cheap LEDs cause a visible flicker due to PWM. This can fail the MOT |
| CANbus compatibility | Modern cars with CANbus systems may show a dashboard warning with LED bulbs. CANbus-compatible LEDs (with built-in resistors) solve this, typically £5 to £10 per pair |
Dave's recommendation: If upgrading to LED, buy branded CANbus-compatible units (Osram, Philips, or NINEO) for £5 to £10. Avoid unbranded £1 to £2 LEDs from eBay. They frequently flicker, fail quickly, or emit a blue tint that fails the MOT.
Cars with Factory LED Number Plate Lights
BMW (most models from 2015+), Audi (most models from 2016+), and Mercedes (most models from 2017+) come with factory LED modules rather than replaceable bulbs. If the LED module fails:
| Car | LED Module Replacement Cost |
|---|---|
| BMW 3 Series | £30 to £60 per unit (OEM), £10 to £20 aftermarket |
| Audi A3/A4 | £25 to £50 per unit (OEM), £8 to £15 aftermarket |
| Mercedes A/C Class | £30 to £55 per unit (OEM), £10 to £20 aftermarket |
These are still easy DIY replacements, the module clips or screws in like a standard housing. No wiring required.
Common Causes of Number Plate Light Failure
Understanding why the light failed helps you prevent recurrence:
1. Bulb End of Life (70% of failures)
Standard W5W halogen bulbs last approximately 1,000 to 2,000 hours. At 2 hours average daily use, that's 1.5 to 3 years. Simple wear, replace and move on.
2. Water Ingress (15% of failures)
The light housing seal degrades over time, allowing water into the housing. This corrodes the bulb contacts and socket, breaking the circuit. Signs: moisture visible inside the housing, corroded or green-tinged contacts.
Fix: Clean the contacts with electrical contact cleaner (£5), apply dielectric grease, and replace the housing seal if available. If the housing itself is cracked, replace the entire unit (£5 to £15 aftermarket).
3. Corroded Wiring Connector (10% of failures)
The electrical connector behind the light housing can corrode, especially on cars exposed to road salt, coastal air, or frequent car washes that spray the rear of the vehicle.
Fix: Disconnect the connector, spray both sides with electrical contact cleaner, scrub gently with a small wire brush, apply dielectric grease, and reconnect. Cost: £5 for contact cleaner and grease.
4. Blown Fuse (5% of failures)
If both number plate lights fail simultaneously, the fuse may have blown. Check your owner's manual for the fuse location (usually in the rear fusebox or under-dash fusebox). Replacement fuses cost under £1.
When You Need a Garage
Most number plate light issues are genuine DIY fixes. You only need a garage if:
- Both lights fail and the fuse is intact (suggests wiring fault, £40 to £80 diagnostic)
- Factory LED module has failed and you're not comfortable with the replacement
- Water ingress has damaged the wiring loom behind the bumper (rare but possible)
Prevention Tips
- Check monthly: Turn on your sidelights, walk behind the car. Takes 5 seconds
- Carry spare bulbs: W5W bulbs cost £1 to £2. Keep a pair in the glovebox. You can fix it at the MOT station if caught out
- Apply dielectric grease to connectors whenever you change a bulb, prevents corrosion
- Replace both bulbs when one fails, the other is likely near end of life too
Related MOT Lighting Guides
Dave covers all common lighting advisories:
- Front fog light not working: DIY or garage
- Headlight aim too low: quick fix guide
- Rear fog light not working: bulb replacement
- Headlight lens cloudy: restoration guide
Check your car's full MOT history for any recurring lighting advisories, repeated number plate light failures suggest a wiring or housing issue worth fixing properly rather than just replacing bulbs repeatedly.