Vehicle Bulb Types Explained: A Complete Reference by Code for Ireland
In this guide
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 Irish road and agricultural vehicles are halogen headlamp bulbs (H-series), bayonet bulbs (P, PY and R series), capless wedge bulbs (W-series), and festoon bulbs (C-series). Most codes also have a trade number (472, 477, 382, 501 and so on) 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 the common vehicle bulb codes used on tractors, agricultural trailers, vans, 4x4s and cars in Ireland. Filament bulb categories are defined in UNECE Regulation 37, and LED light sources in UNECE Regulation 128. Because Ireland is in the EU, bulbs and lamps carrying EU “e” marks or UNECE “E” marks from any country are recognised. The trade numbers are a long-established UK numbering system printed on the box by most UK bulb brands and ag-parts wholesalers, including the bulbs we ship to Ireland.
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 halogen category. 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.
Signal bulb series (P, PY, R, W, T, C codes). The letters name the category rather than spelling out the base, but there is a pattern. P and R bulbs mostly sit on metal bayonet caps (P21W and R5W on BA15s), W bulbs on glass wedge bases, C bulbs are festoons and T4W is a small tubular bulb on a BA9s bayonet. P27W is an exception: it is a wedge bulb. The number gives the wattage and the final W stands for watts (P21W = 21 watts). A “Y” in the letter group means amber glass (PY21W) and an “R” after the first letter means red glass (PR21W). A slash separates the two wattages of a twin-filament bulb (P21/5W = 21W brake filament and 5W tail filament).
Trade number (catalogue shorthand). The trade number is a three-digit number used in parts catalogues. 472 = H4. 477 = H7 (some brands use 499). 382 = P21W. 380 = P21/5W. 501 = W5W. 581 = PY21W. The 24V version of a bulb has its own number, for example 241 for a 24V P21W.
The same bulb often appears in catalogues under two labels at once: the ECE category (H7) and the trade number (477). Both describe the same bulb, so matching either one, plus the voltage, confirms 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 Irish road and ag vehicles are listed below.
| ECE code | Trade number | Base | Filaments | Wattage 12V | Use |
|---|---|---|---|---|---|
| H1 | 448 | P14.5s | 1 | 55W | Main beam, fog, spot, work lamp |
| H3 | 453 | PK22s | 1 | 55W | Fog lamp, work lamp |
| H4 | 472 | P43t | 2 | 60/55W | Combined dip + main |
| H7 | 477 (some brands 499) | PX26d | 1 | 55W | Dip OR main (twin-bulb) |
| H8 | 708 | PGJ19-1 | 1 | 35W | Fog lamp |
| H11 | 711 | PGJ19-2 | 1 | 55W | Fog, dip, auxiliary |
| H15 | 715 | PGJ23t-1 | 2 | 55/15W | DRL + main beam |
H4 and H7 are the most common halogen tractor headlamp bulbs, with H1 and H3 widely used in work lamps and auxiliary lamps. H8 and H11 appear in fog lamps and some headlamps on newer vehicles. H15 appears mostly on cars and vans with a combined daytime running lamp and main beam. The trade numbers for H8, H11 and H15 are less standardised than the older codes, so order these by ECE code. The two most common headlamp bulbs are compared in H4 vs H7 bulbs.
The common H-codes also come in 24V versions with the same base and filament arrangement but a higher wattage. The 24V H1, H3, H7 and H11 are rated 70W, and the 24V H4 is rated 75/70W (trade number 475). Tractors and HGVs with 24V electrical systems must use 24V bulbs.
The fog and spot bulb with a plastic base used in many auxiliary lamps is covered in 880 and 881 (H27W) bulbs explained. For converting headlamps, see LED headlamp conversions.
Bayonet Bulbs (P, PY, R and T Series)
Bayonet bulbs use a metal base with two locating pins that twist into a slotted socket. The position of the pins and the number of contacts on the bottom tell you which bulb fits. The most common bayonet bulbs on Irish road and ag vehicles are listed below.
| ECE code | Trade number 12V / 24V | Base | Filaments | Wattage | Use |
|---|---|---|---|---|---|
| P21W | 382 / 241 | BA15s | 1 | 21W | Indicator, reverse, rear fog, brake |
| PY21W | 581 / 588 | BAU15s | 1 | 21W amber | Indicator (amber) |
| P21/5W | 380 / 294 | BAY15d | 2 | 21W + 5W | Brake + tail combined |
| P21/4W | 566 / 567 | BAZ15d | 2 | 21W + 4W | Brake + tail (rare) |
| R5W | 207 / 149 | BA15s | 1 | 5W | Tail, position, number plate |
| R10W | 245 / 246 | BA15s | 1 | 10W | Tail, interior |
| T4W | 233 / 249 | BA9s | 1 | 4W | Position, side marker, panel |
| H6W | 434 | BAX9s | 1 | 6W | Sidelight, position |
| H21W | 435 | BAY9s | 1 | 21W | Reverse, fog, indicator (compact) |
BA15s and BAY15d are the two most common bayonet bases on agricultural rear lamps. A standard tractor rear lamp cluster typically uses P21W (382) or PY21W (581) for indicators, P21/5W (380) for brake and tail combined, and R5W (207) or R10W (245) for the 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 Ireland and across Europe. The amber colour is in the glass, which means a clear lens with a PY21W produces the correct amber flash. Its BAU15s base has the pins offset (about 150 degrees apart instead of 180), so it cannot be swapped by mistake with a clear P21W.
For replacement bulb specifics on rear clusters, see rear brake light bulbs.
Capless Wedge Bulbs (W-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” prefix identifies the wedge base. The number after gives the wattage.
| ECE code | Trade number 12V / 24V | Base | Filaments | Wattage | Use |
|---|---|---|---|---|---|
| W5W | 501 / 507 | W2.1×9.5d | 1 | 5W | Sidelight, position, number plate, dash |
| W3W | 504 / 505 | W2.1×9.5d | 1 | 3W | Position, gauge |
| W16W | 955 (often sold as 921) | W2.1×9.5d | 1 | 16W | Reverse, high-level brake |
| W21W | 582 | W3×16d | 1 | 21W | Reverse, indicator (newer) |
| W21/5W | 580 | W3×16q | 2 | 21W + 5W | Tail + brake (newer cars) |
| WY21W | 585 | WX3×16d | 1 | 21W amber | Indicator (newer) |
| P27W | 182 | W2.5×16d | 1 | 27W | Indicator, brake (some imported vehicles) |
| T10 LED | (501 LED) | W2.1×9.5d | LED | various | LED replacement for W5W |
W5W (trade number 501) is one of the most widely fitted bulbs on any vehicle. It sits in front and rear sidelights, position lamps, dashboard gauges, glove-box lamps and number plate lamps. The trade number 501 is understood well beyond the UK.
T10 is used loosely to describe both the W5W and its LED equivalents. It refers to the roughly 10 mm diameter of the glass. 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 wedge alternative to PY21W used in many newer cars and vans with capless rear lamp clusters. Agricultural lamps more often use bayonet bulbs.
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 type. Length matters as much as wattage, so measure the old bulb.
| ECE code | Trade number | Size | Filaments | Wattage 12V | Use |
|---|---|---|---|---|---|
| C5W | 239 | 11 x 38 mm (approx) | 1 | 5W | Interior, number plate |
| 10W festoon (often sold as C10W) | 272 or 264 | 11 x 38 mm (272), 11 x 44 mm (264) | 1 | 10W | Interior, courtesy |
Festoon bulbs sit mostly in interior cabin lamps, courtesy lamps and a few legacy number plate lamps on older trailers. Many newer designs use W5W bulbs or LED units instead, partly because festoons can 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
Most common bulb codes have LED retrofit bulbs designed to drop into the original socket. Most of these carry no approval as a replacement for the filament bulb. Since 2025 a small number of LED replacement bulbs have been approved under Regulation 37, starting with the H11 category; at the time of writing none is approved for H4 or H7.
LED retrofit bulbs are sold under the original ECE code or trade number with “LED” added: H7 LED, P21W LED, W5W LED, 472 LED. The drop-in fit means buyers tend to choose by socket and function, so check the approval position for the lamp it is going into.
Two LED-specific issues affect retrofit choice:
CANbus compatibility. Many modern vehicles, mostly cars and vans but also some newer tractors, monitor bulb current draw to spot blown filaments. LED retrofits draw far less current than halogen and can trigger false “bulb out” warnings. CANbus-compatible LEDs include load resistors or smart drivers that mimic the filament draw and avoid the warning. In indicators, a low-draw LED can also make the flasher unit flash too fast, which needs an LED-compatible flasher or a load resistor.
Polarity. Many LED retrofits are polarity-sensitive. If a capless LED stays dark, turn it round in the socket. Halogen filaments do not care about polarity; LEDs do.
For headlamps the Irish position comes from RSA guidance on HID and LED lights. Retrofit LED or HID bulbs are not always compatible with the vehicle or the headlamp. An LED bulb is legal only if it is type-approved, fitted to a compatible vehicle, and its beam does not dazzle. A vehicle fails the NCT (or the CVRT for commercial vehicles) if it has an unapproved kit, an approved kit on an incompatible vehicle, or HID bulbs retrofitted into ordinary halogen headlamps. Complete type-approved LED headlamp units, which replace the whole lamp, avoid this problem. In rear and signal lamps, any fitted lamp must work and show the correct colour, so an LED bulb must give the right colour and enough light through the lens. If in doubt, check current RSA guidance before converting.
For technology background, see LED vs halogen tractor lights.
Quick Trade Number Reference Table
One of the most useful references for any ag vehicle owner is the trade number lookup. The table below covers the most common trade numbers on tractors, agricultural trailers, vans and 4x4s.
| Trade number | ECE code | Voltage | Wattage | Function |
|---|---|---|---|---|
| 207 | R5W | 12V | 5W | Tail, position |
| 233 | T4W | 12V | 4W | Side marker |
| 239 | C5W | 12V | 5W | Interior festoon |
| 241 | P21W | 24V | 21W | Indicator, reverse, brake |
| 245 | R10W | 12V | 10W | Tail, interior |
| 264 / 272 | 10W festoon | 12V | 10W | Interior festoon |
| 294 | P21/5W | 24V | 21/5W | Brake + tail |
| 380 | P21/5W | 12V | 21/5W | Brake + tail |
| 382 | P21W | 12V | 21W | Indicator, reverse, brake |
| 434 | H6W | 12V | 6W | Sidelight (BAX9s base) |
| 435 | H21W | 12V | 21W | Reverse, fog (BAY9s base) |
| 448 | H1 | 12V | 55W | Main beam, work lamp halogen |
| 453 | H3 | 12V | 55W | Fog/spot halogen |
| 472 | H4 | 12V | 60/55W | Dip + main halogen |
| 474 | H7 | 24V | 70W | Single-filament halogen |
| 475 | H4 | 24V | 75/70W | Dip + main halogen |
| 477 (or 499) | H7 | 12V | 55W | Single-filament halogen |
| 501 | W5W | 12V | 5W | Sidelight, position |
| 504 | W3W | 12V | 3W | Position, gauge |
| 507 | W5W | 24V | 5W | Sidelight, position |
| 566 | P21/4W | 12V | 21/4W | Brake + tail (rare) |
| 580 | W21/5W | 12V | 21/5W | Tail + brake (newer) |
| 581 | PY21W | 12V | 21W amber | Indicator amber |
| 582 | W21W | 12V | 21W | Reverse (newer) |
| 880 / 881 | H27W/1 / H27W/2 | 12V | 27W | Fog, spot |
| 955 | W16W | 12V | 16W | Reverse, high-level brake |
The trade number is a quick way to order a bulb because counter staff, suppliers and parts catalogues understand it, and our team will recognise it if you ring us on +44 1522 440522. Always check the voltage as well, because 12V and 24V versions of the same bulb have different numbers but look identical. 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), the voltage and the wattage. Trade numbers (472, 477, 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 or send us a photo at [email protected]. Counter staff can identify most bulbs quickly by base shape, contact count 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, PY, R and T series), capless wedge (W-series), and festoon (C-series). Every code locks the base, filament count and wattage, and most codes have a trade number (472, 477, 382, 501 and so on) used in parts catalogues, with separate numbers for 24V versions. LED retrofits exist for most codes, but very few carry approval as a filament replacement, and RSA guidance says unapproved or incompatible LED and HID conversions in headlamps fail the NCT. 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, including standard halogen headlamp bulbs and standard miniature signal bulbs. Orders ship from Lincolnshire, with free delivery to Ireland on orders over €150.
