Rear Brake Light Bulbs: P21W, P21/5W, Fitting and UK Law

A rear brake light bulb is the 21W bulb that lights up red when the brake pedal is pressed, warning following traffic that the vehicle is slowing or stopping. Most UK cars, vans, tractors and trailers use either a P21W (382) single-filament bulb or a P21/5W (380) twin-filament bulb that also runs the tail light.

This guide covers both bulb types, how to tell which one you need, LED replacements, how to change the bulb, and what the law and the MOT require. For tractors, including the speed and age exemptions, brake light switch faults and independent brake pedals, see our dedicated guide to tractor brake lights. Each bulb also has its own guide: the P21W 382 bulb and the P21/5W 380 bulb.

What Does a Rear Brake Light Bulb Do

A rear brake light bulb illuminates when the driver presses the brake pedal, producing a bright red warning that tells following traffic the vehicle is slowing down or stopping. Most vehicles on UK roads carry at least 2 rear brake lights to communicate this single, critical piece of information: “I am braking.”

The rear brake light bulb operates on a simple circuit. A switch on the brake pedal (or a pressure switch in the brake hydraulics) closes the circuit when the brake is applied, sending current to the bulbs. Release the pedal and the circuit breaks. There is no dimming, no delay, and no manual switch.

A rear brake light bulb is rated at 21W, roughly 4 times the 5W of the rear position lamp (the dim red glow visible whenever the vehicle’s lights are on). A following driver needs to distinguish instantly between a vehicle ahead with its lights on and a vehicle that is actively braking. The difference in brightness provides that distinction.

Rear brake lights are separate from position lamps (continuous dim red for visibility), reverse lights (white, engaged in reverse gear), and rear fog lights (bright red, switched on manually in poor visibility). These functions often share the same rear lamp cluster, but each has its own bulb or filament and its own circuit.

Rear Brake Light Bulb Types and Codes

The 2 most common rear brake light bulbs in UK vehicles are the P21W (trade code 382) and the P21/5W (trade code 380). Between them, these 2 bulbs cover the brake light function on the vast majority of cars, vans, tractors, and trailers on the road today.

P21W (Trade Code 382)

The P21W is a single-filament bulb rated at 21W. It handles the brake light function only. The base type is BA15s, which is a 15mm bayonet base with a single electrical contact at the bottom and 2 locating pins set level with each other on opposite sides. The P21W pushes into the socket and twists clockwise to lock.

The P21W is used in vehicles where the brake light and the rear position lamp are housed in separate sections of the rear lamp cluster, each with its own socket.

P21/5W (Trade Code 380)

The P21/5W is a dual-filament bulb with 2 separate filaments inside a single glass envelope. The thicker filament runs at 21W and powers the brake light. The thinner filament runs at 5W and powers the rear position lamp. The base type is BAY15d, a 15mm bayonet base with 2 electrical contacts at the bottom (one for each filament) and 2 locating pins set at different heights.

The offset pins ensure the bulb fits into the socket in only 1 orientation, connecting the 21W filament to the brake circuit and the 5W filament to the position lamp circuit. If the bulb could go in either way round, the brake and tail filaments could be swapped, giving a bright tail light and a weak brake light.

The P21/5W is used in vehicles where a single bulb serves both the brake light and the rear position lamp. This combined arrangement is the most common setup across cars, vans, and agricultural vehicles.

Quick Reference Table

Bulb Type	Trade Code	Filaments	Wattage	Base	Function
P21W	382	1	21W	BA15s (single contact, level pins)	Brake light only
P21/5W	380	2	21W + 5W	BAY15d (dual contact, offset pins)	Brake light + position lamp
PR21W	n/a	1	21W	BAW15s (angled pins)	Brake light (red glass)

The PR21W is a red-glass version of the P21W, used in rear lamp clusters that have a clear or smoked outer lens where the bulb itself provides the red colour. Its angled pins stop it being swapped with a clear or amber bulb.

Single Filament vs Dual Filament Brake Light Bulbs

A single-filament rear brake light bulb (P21W) handles braking only, while a dual-filament bulb (P21/5W) combines the brake light and the rear position lamp in 1 bulb. The choice between them depends entirely on the design of the vehicle’s rear lamp cluster.

How to Tell Which Type Your Vehicle Uses

Remove the existing rear brake light bulb and look at it. A single-filament bulb has 1 filament inside the glass, 1 contact on the base and 2 pins at the same height. A dual-filament bulb has 2 filaments inside the glass, 2 contacts on the base and 2 offset (staggered) pins. The vehicle handbook also lists the bulb type for every lamp position, referenced by ECE code (P21W, P21/5W) or trade code (382, 380).

Why the Distinction Matters

Fitting a single-filament P21W into a dual-filament P21/5W socket does not work properly. The brake light may function, but the rear position lamp will not illuminate because the P21W has no second filament. A P21/5W will not seat properly in a P21W holder because of its offset pins and second contact, so never force it.

On vehicles with combined brake and tail light units (the majority), the rear brake light bulb is a P21/5W. On vehicles with separate brake and tail light sections, the rear brake light bulb is a P21W.

LED vs Halogen Rear Brake Light Bulbs

LED rear brake lights respond faster than filament bulbs because they have no filament to heat up before producing light. The difference is commonly put at up to about 0.2 seconds. At 100 km/h (roughly 62 mph), 0.2 seconds is about 5.5 metres of travel for the driver behind. On a country road with a tractor pulling out of a field gateway, those metres matter.

Advantages of LED Brake Lights

LED rear brake lights offer 4 practical advantages over filament bulbs:

1. Faster response time. An LED reaches full brightness almost instantly. A filament takes a fraction of a second to heat up and glow.
2. Longer lifespan. A quality LED lamp lasts many times longer than a 21W filament bulb, which matters on a vehicle that sees daily use.
3. Lower power draw. An LED stop light typically draws a few watts instead of 21W. This reduces the load on the vehicle’s electrical system, which is particularly relevant on tractors and older vehicles with limited alternator capacity.
4. Vibration resistance. LEDs have no filament to break. Agricultural vehicles, trailers bouncing along farm tracks, and machinery operating on rough ground all subject bulbs to constant vibration. A filament is a thin wire suspended in a glass envelope. It breaks. An LED does not.

CANBUS Compatibility

Some vehicles (particularly modern cars with onboard diagnostics) monitor the current draw of each bulb circuit. When an LED draws a few watts instead of the expected 21W, the vehicle’s computer interprets the low current as a blown bulb and triggers a dashboard warning. This is a CANBUS or bulb-monitoring error, not a fault with the LED.

CANBUS-compatible LED bulbs include a built-in resistor that mimics the current draw of a filament bulb. Older tractors and most agricultural trailers have no bulb monitoring, so standard LED lamps usually work without warnings on farm vehicles. Some newer tractors do monitor lamp circuits, so check before buying.

Legal Position

Factory-fitted LED brake lights and complete aftermarket LED rear lamps that carry an approval mark are fully road legal. Retrofit LED bulbs (an LED pushed into a lamp designed for a filament bulb) are a different matter: the lamp was approved with a filament bulb, and the LED bulb changes how it performs. The MOT fails a stop lamp that does not show a steady red light, has significantly reduced intensity, or does not meet the position and intensity requirements. If you want LED brake lights, a complete approved LED rear lamp is the reliable route.

How to Replace a Rear Brake Light Bulb

Replacing a rear brake light bulb takes 5 to 10 minutes on most vehicles and needs little more than a screwdriver and a clean cloth. The procedure is the same for cars, vans, and tractors with enclosed rear lamp clusters.

Step-by-Step Guide

Step 1: Identify the correct bulb. Check the vehicle handbook or remove the existing bulb and read the code printed on the base. For most vehicles, the rear brake light bulb is a P21W (382) or P21/5W (380) at 12V. Some tractors and plant run 24V.

Step 2: Access the rear lamp cluster. On cars and vans, access is usually from inside the boot or tailgate. Open the boot and peel back the trim panel to expose the bulb holders. On some vehicles, the entire rear lamp unit unbolts from the outside (2 or 3 bolts). On tractors, rear lamp units are typically surface-mounted with the lens held on by 2 screws.

Step 3: Remove the bulb holder. Twist the bulb holder anticlockwise (roughly a quarter turn) and pull it straight out of the lamp cluster. The bulb comes out with the holder.

Step 4: Remove the old bulb. For a bayonet bulb (P21W or P21/5W), push the bulb in slightly, twist anticlockwise, and pull it out. For a wedge bulb, pull it straight out.

Step 5: Fit the new bulb. Push the new bulb into the holder and twist clockwise to lock. For a P21/5W, the offset pins ensure correct orientation. Do not force it. If the pins do not align, rotate the bulb 180 degrees.

Step 6: Refit and test. Push the bulb holder back into the lamp cluster and twist clockwise to lock. Ask someone to press the brake pedal while you watch the light from behind. If the vehicle uses a P21/5W dual-filament bulb, also check that the tail light (dim red) works with the sidelights turned on. Both filaments need to function.

Common Mistakes

Avoid touching the glass of a bulb with bare fingers. Oils from skin create hot spots that can shorten its life. Handle the bulb by its metal base or use a clean cloth.

On a dual-filament P21/5W, the brake filament can fail while the tail filament continues to work. The tail light glows normally, but the brake light does nothing. Always test both functions separately.

UK Legal Requirements for Rear Brake Lights

UK law requires 2 rear brake lights (stop lamps) on most motor vehicles and trailers first used or made on or after 1 January 1971, and 1 on older vehicles. The rules sit in the Road Vehicles Lighting Regulations 1989 (RVLR 1989), mainly Schedule 1 and Schedule 12.

What the Law Requires

Obligatory rear brake lights must meet the following requirements:

• Quantity: 2, 1 on each side of the centre line, at least 400 mm apart and forming a pair
• Height: between 350 mm and 1,500 mm from the ground (up to 2,100 mm where the vehicle structure makes 1,500 mm impracticable)
• Colour: red
• Operation: must be operated by the service brake, and on a trailer by the towing vehicle’s service brake
• Marking: an approval mark on vehicles first used from 1 February 1974
• High-level brake light: an optional extra stop lamp may be fitted. On vehicles first used on or after 1 April 1991, a centrally mounted stop lamp must give between 20 and 60 candelas from directly behind. Most cars built since the late 1990s have one as standard.

Regulation 23 of the RVLR also requires every stop lamp fitted to a vehicle, including an optional high-level lamp, to be kept in good working order.

Exemptions

Vehicles with a maximum speed not exceeding 25 mph, agricultural vehicles and works trucks first used before 1 April 1986, and vehicles first used before 1 January 1936 do not have to carry stop lamps. Trailers drawn by agricultural vehicles are also exempt. Any stop lamps that are fitted must still work.

MOT Testing

The MOT test checks every rear brake light for operation, colour, and condition. Under the DVSA MOT inspection manual, all stop lamps must light immediately when the brake is applied and go out immediately when it is released, and they must show a steady red light.

The tester checks for:

• A stop lamp that does not work, or does not show a steady red light, or has significantly reduced intensity (a major defect)
• A stop lamp switch with a delay in operation (major)
• Stop lamps that stay on when the brake is released (dangerous)
• Lens condition (a defective lens that affects the light is a major defect)
• Secure mounting (the lamp unit must not be loose or at risk of detaching)

Penalties

Driving with a faulty rear brake light is an offence under the RVLR. The police may issue a fixed penalty, or offer a vehicle defect rectification notice that requires the fault to be fixed and proof provided within a set time. A vehicle in a dangerous condition can also be stopped from continuing its journey.

For agricultural vehicles on unlit rural roads, a following driver may have no other visual cue that the vehicle ahead is braking.

Rear Brake Light Bulbs on Tractors and Agricultural Trailers

Tractors and agricultural trailers use the same rear brake light bulb types as cars and vans, but fitment differs because of exposed lamp housings, higher vibration, and the choice between 12V and 24V electrical systems.

Tractor Brake Lights

Most tractors run 12V electrical systems and use standard P21W or P21/5W rear brake light bulbs. Some larger agricultural machines and plant run 24V. A 12V rear brake light bulb fitted to a 24V circuit will burn out almost immediately. A 24V bulb fitted to a 12V circuit will glow dimly and produce insufficient light for legal compliance.

Check the voltage before buying replacements. The voltage is printed on the bulb base (12V or 24V) and listed in the tractor’s operator manual.

A tractor is exempt from the brake light requirement if it was first used before 1 April 1986, or if its maximum speed does not exceed 25 mph (which includes 40 km/h tractors). Most tractors have brake lights fitted anyway, and once fitted they must work. The tractor brake lights guide linked at the top of this page explains the rules and covers switch faults in detail.

Agricultural Trailer Brake Lights

Agricultural trailers connect their rear brake light bulbs to the towing vehicle through the towing socket. On both the 7-pin (12N) socket and the 13-pin socket, the brake light circuit is pin 6 (red cable). The trailer brake light illuminates whenever the tractor driver presses the brake pedal, provided the wiring is intact and the socket contacts are clean.

Trailer rear lamp units take significant punishment from road spray, loading impacts, and power washing. Corrosion in the bulb socket or the trailer plug is one of the most common causes of brake light failure on agricultural trailers, often more so than the bulb itself failing.

Practical Tips for Farm Vehicles

Keep a spare P21W and a spare P21/5W rear brake light bulb in the tractor cab. They take minutes to fit. A walk-around check before any road journey catches failed brake lights before they become an offence. On trailers, clean the towing socket contacts with electrical contact cleaner at the start of each season and apply dielectric grease to prevent corrosion.

LED rear lamps are a particularly good fit for agricultural trailers. No filament means vibration from rough tracks does not cause premature failure. The lower power draw reduces the load on the tractor’s alternator. And the faster response time gives following traffic extra warning, which matters when a loaded grain trailer takes a long time to stop.

Browse rear lamps at Agri Lighting for complete rear lamp units and LED upgrades, or our bulb range for replacement P21W and P21/5W bulbs.

Frequently asked questions

Is a brake light bulb a 380 or a 382?

It depends on the lamp. A 382 (P21W) is a single-filament bulb used where the brake light has its own socket. A 380 (P21/5W) is a twin-filament bulb that runs both brake and tail lights. Check the old bulb: 1 contact on the base means 382, 2 contacts and offset pins mean 380.

Why is my brake light not working when the bulb is fine?

If both brake lights are out, suspect the brake light switch or fuse. If 1 side is out with a good bulb, check the bulb holder for corrosion and the earth connection. On a trailer, check pin 6 in the plug and socket.

Can I fit LED brake light bulbs?

LED replacement bulbs fit the same sockets, but a lamp designed for a filament bulb was approved with one. The safest legal upgrade is a complete LED rear lamp carrying an approval mark. Cars with bulb monitoring may also need CANBUS-compatible LEDs.

Is it illegal to drive with a broken high-level brake light?

Yes. Every stop lamp fitted to a vehicle, including an optional high-level lamp, must be kept in good working order under regulation 23 of the RVLR, so a dead high-level brake light should be fixed as soon as possible.