A vehicle bulb is identified by an alphanumeric code that fixes its base shape, filament arrangement, wattage and rated voltage. The four most common families of code on UK road and agricultural vehicles are halogen headlamp bulbs (H-series), bayonet bulbs (P-series), capless wedge bulbs (W-series and T-series), and festoon bulbs (C-series). Each code has a corresponding two- or three-digit UK trade code (472, 499, 382, 501 etc.) used in parts catalogues. Matching the code is the difference between a five-minute fix and a wrong bulb that does not seat or fails on first start.
This reference covers every common vehicle bulb code used on UK tractors, agricultural trailers, vans, 4x4s and cars. Codes follow ECE Regulations 37 and 128. UK trade codes follow the long-established Lucas catalogue numbering still used by Halfords, Euro Car Parts, GSF, Powerbulbs and most ag-parts wholesalers.
How Vehicle Bulb Codes Work
A vehicle bulb code identifies four properties at once: the base shape, the filament count, the wattage, and the technology. The format follows three patterns depending on the bulb family.
Halogen headlamp series (H-codes). A letter “H” followed by a number identifies a single ECE Regulation 37 filament arrangement. H1, H3, H4, H7, H8, H11 and H15 are the common ag and road examples. Each H-code locks down the base, filament position and wattage.
Bayonet and capless series (P, PY, W, T, C codes). Letters identify the base type (P for bayonet, W for wedge, C for festoon, T for tubular). The wattage follows immediately afterwards in watts (e.g., P21W = bayonet 21 watt). A “Y” inside the letter group indicates an amber or yellow lens (PY21W = bayonet 21 watt yellow). A slash separates dual-filament wattages (P21/5W = bayonet bulb with 21W brake filament and 5W tail filament).
Trade code (UK shorthand). The Lucas trade code is a two- or three-digit number used in parts catalogues. 472 = H4. 499 = H7. 382 = P21W. 501 = W5W. 581 = PY21W. The trade code points to the same physical bulb as the ECE code.
The same bulb often appears in catalogues under three labels at once: ECE code (H7), ICAO short code (also H7), and trade code (499). All three describe the same bulb. Buyers should match any one of the three to confirm the right part.
For halogen headlamp specifics, see halogen headlamp bulb types explained.
Halogen Headlamp Bulbs (H-Series)
H-series bulbs are halogen filament headlamp bulbs governed by ECE R37. Each code locks the base, filament count and wattage. The seven most common H-codes on UK road and ag vehicles are listed below.
| ECE code | Trade code | Base | Filaments | Wattage 12V | Use |
|---|---|---|---|---|---|
| H1 | 448 | P14.5s | 1 | 55W | Main beam, fog, spot |
| H3 | 453 | PK22s | 1 | 55W | Fog lamp, work lamp |
| H4 | 472 | P43t | 2 | 60/55W | Combined dip + main |
| H7 | 499 | PX26d | 1 | 55W | Dip OR main (twin-bulb) |
| H8 | 64212 | PGJ19-1 | 1 | 35W | Fog lamp |
| H11 | 64211 | PGJ19-2 | 1 | 55W | Fog, dip, auxiliary |
| H15 | 64176 | PGJ23t-1 | 2 | 55/15W | DRL + main beam |
H1, H4 and H7 cover most UK tractor headlamps from 1985 to date. H8 and H11 appear in fog lamps on newer 4x4s and some tractors built after 2010. H15 appears mostly on cars and vans with combined daytime running lamp and main beam.
A 24V version exists for every H-code. The 24V variant has the same base and filament arrangement but rated at 70W or higher to deliver the same lumen output as the 12V 55W version. Tractors and HGVs with 24V electrical systems must use 24V bulbs.
For the wider headlamp guide, see halogen headlamp bulb types and LED headlamp conversions.
Bayonet Bulbs (P-Series)
Bayonet bulbs use a metal base with one or two locating pins that twist into a slotted socket. The “P” prefix identifies the bayonet base. Wattage follows the prefix. The most common bayonet bulbs on UK road and ag vehicles are listed below.
| ECE code | Trade code | Base | Filaments | Wattage 12V | Use |
|---|---|---|---|---|---|
| P21W | 382 | BA15s | 1 | 21W | Indicator, reverse, brake |
| PY21W | 581 | BAU15s | 1 | 21W amber | Indicator (amber) |
| P21/5W | 380 | BAY15d | 2 | 21W + 5W | Brake + tail combined |
| P21/4W | 566 | BAZ15d | 2 | 21W + 4W | Brake + tail (rare) |
| P27W | 4114 | W2.5x16d | 1 | 27W | High-output indicator/brake |
| R5W | 207 | BA15s | 1 | 5W | Tail, position |
| R10W | 245 | BA15s | 1 | 10W | Tail, interior |
BA15s and BAY15d are the two most common bayonet bases on agricultural rear lamps. A standard tractor rear lamp cluster typically uses P21W (382) for indicators, P21/5W (380) for brake and tail combined, and R10W (245) for number plate or interior. Many older agricultural trailers still use these exact bulbs in 12V or 24V form.
The “Y” in PY21W indicates an amber bulb. PY21W is the standard amber indicator bulb in the UK and across Europe. The amber tint is in the glass, not in the lens, which means a clear lens with a PY21W produces the correct amber flash for indicators.
For replacement bulb specifics on rear clusters, see rear brake light bulbs.
Capless Wedge Bulbs (W-Series and T-Series)
Capless wedge bulbs have no metal base. The wires fold up from the bulb body and the bulb pushes directly into a flexible socket. The “W” or “T” prefix identifies the wedge base. The number after gives the wattage.
| ECE code | Trade code | Base | Filaments | Wattage 12V | Use |
|---|---|---|---|---|---|
| W5W | 501 | W2.1×9.5d | 1 | 5W | Sidelight, position, dash |
| W3W | 504 | W2.1×9.5d | 1 | 3W | Position, gauge |
| W21W | 582 | W3x16d | 1 | 21W | Reverse, indicator (newer) |
| W21/5W | 580 | W3x16q | 2 | 21W + 5W | Tail + brake (newer cars) |
| WY21W | 582-1 | W3x16d | 1 | 21W amber | Indicator (newer) |
| T4W | 233 | BA9s | 1 | 4W | Position, side marker |
| T10 | (501 LED) | W2.1×9.5d | LED | various | LED replacement for W5W |
W5W (trade code 501) is the single most-fitted wedge bulb in the world. It sits in front and rear sidelights, position lamps, dashboard gauges, glove-box lamps and number plate lamps on millions of UK vehicles. The trade code 501 is shorthand even outside the UK.
T10 is sometimes used loosely to describe both the W5W and its LED equivalents. Strictly, T10 is the lamp envelope size (10/8 inch diameter, “T-ten”). A T10 LED bulb fits the same socket as a W5W and is a common LED upgrade for sidelight and number plate use.
WY21W is a newer wedge replacement for PY21W in cars and vans built after 2005 with capless rear lamp clusters. It is rare on agricultural vehicles, which still use bayonet bases for durability.
Festoon Bulbs (C-Series)
Festoon bulbs are tubular, with a metal tip at each end that contacts a sprung holder. The “C” prefix identifies the festoon base. Length follows in millimetres. Two common UK trade codes appear in road and ag vehicles.
| ECE code | Trade code | Length | Filaments | Wattage 12V | Use |
|---|---|---|---|---|---|
| C5W | 239 | 36 to 39 mm | 1 | 5W | Interior, number plate |
| C10W | 264 | 36 to 41 mm | 1 | 10W | Interior, courtesy |
Festoon bulbs sit mostly in interior cabin lamps, courtesy lamps and a few legacy number plate lamps on older trailers. Modern designs have moved to W5W or LED equivalents because festoons tend to vibrate loose under farm shock loads. A festoon bulb that keeps falling out is a common reason to upgrade an interior lamp to a sealed LED unit.
LED Equivalents and CANbus-Compatible Bulbs
Every common ECE bulb code has at least one LED equivalent designed to drop into the original socket. LED equivalents are not separately ECE-coded under R37 because R37 governs filament lamps; LED retrofits sit under different approval pathways or, more commonly, fall outside formal type approval.
LED retrofit bulbs are sold under the original ECE or trade code with “LED” added: H7 LED, P21W LED, W5W LED, 472 LED. The drop-in compatibility means the buyer chooses by socket and function, not by approval status.
Two LED-specific issues affect retrofit choice:
CANbus compatibility. Modern vehicles (mostly cars built after 2005, some tractors built after 2015) monitor bulb current draw to spot blown filaments. LED retrofits draw far less current than halogen and trigger false “bulb out” warnings. CANbus-compatible LEDs include load resistors or smart drivers that mimic the filament draw and avoid the warning.
Polarity. Many LED retrofits are polarity-sensitive. If the LED stays dark, reverse the connector. Halogen filaments do not care about polarity; LEDs do.
For the UK road legality position on LED retrofit bulbs in halogen headlamps, see LED headlamp conversions. For LED retrofits in rear lamp clusters, the legal position is generally clearer because rear lamp ECE approvals (R7, R6, R23) have specific LED-class versions that some retrofit bulbs hold.
For technology background, see LED vs halogen tractor lights.
Quick Trade Code Reference Table
The single most useful reference for any UK ag vehicle owner is the trade code lookup. The table below covers the most common UK trade codes on tractors, agricultural trailers, vans and 4x4s.
| Trade code | ECE code | Wattage | Function |
|---|---|---|---|
| 207 | R5W | 5W | Tail, position |
| 233 | T4W | 4W | Side marker |
| 239 | C5W | 5W | Interior festoon |
| 245 | R10W | 10W | Tail, interior |
| 264 | C10W | 10W | Interior festoon |
| 380 | P21/5W | 21/5W | Brake + tail |
| 382 | P21W | 21W | Indicator, reverse, brake |
| 434 | H21W | 21W | Indicator (BAY9s base) |
| 448 | H1 | 55W | Main beam halogen |
| 453 | H3 | 55W | Fog/spot halogen |
| 472 | H4 | 60/55W | Dip + main halogen |
| 499 | H7 | 55W | Single-filament halogen |
| 501 | W5W | 5W | Sidelight, position |
| 504 | W3W | 3W | Position, gauge |
| 566 | P21/4W | 21/4W | Brake + tail (rare) |
| 580 | W21/5W | 21/5W | Tail + brake (newer) |
| 581 | PY21W | 21W amber | Indicator amber |
| 582 | W21W | 21W | Reverse (newer) |
The trade code is the fastest way to buy a bulb in the UK because every counter assistant, vendor and parts catalogue understands it. ECE codes are correct but more abstract for first-time buyers.
How to Identify the Bulb You Need
Three identification routes work for any vehicle bulb. The fastest is the bulb itself; the second is the lamp body; the third is the operator’s manual.
Route 1: read the bulb. Pull the existing bulb out of its socket. Look at the metal base or the glass collar. Most bulbs are stamped with the ECE code (H7, P21W, W5W) and the wattage. UK trade codes (472, 499, 382, 501) appear on the box but rarely on the bulb itself.
Route 2: read the lamp body. Many lamp housings have the bulb code moulded into the inside of the unit. Open the lamp by removing the lens or the rear cover. Look for a small embossed label near the bulb socket.
Route 3: check the operator’s manual. Tractor and vehicle manuals list every bulb code in the lighting section. For tractors more than 25 years old, the manual is sometimes the only reliable source because the bulb itself may have been replaced with a non-original part.
For vehicles where none of the three routes give a clear answer, take the old bulb to a parts counter. A counter assistant identifies most bulbs in 30 seconds by base shape and filament count.
For a tractor-by-tractor fitment list, see the tractor light fitment guide and the brand-specific guides for John Deere, New Holland, Massey Ferguson and Case IH.
Vehicle Bulb Type Summary
The vehicle bulb code system covers four families: halogen headlamp (H-series), bayonet (P-series), capless wedge (W-series and T-series), and festoon (C-series). Every code locks the base, filament count and wattage, and every code has a UK trade equivalent (472, 499, 382, 501 and so on) used in parts catalogues. LED retrofits exist for every code but sit outside the original ECE filament approval. The fastest identification path is to read the bulb itself, the second is to read the lamp body, the third is the operator’s manual. Match the code, the voltage and the wattage on every replacement and a five-minute job stays a five-minute job.
For replacement bulbs across all categories, see the Agri Lighting bulb range, which covers H-series, P-series, W-series and festoon types in 12V and 24V variants.