Guides

The background to the answers on our car pages: what the codes mean, what the law says, and how to check and fit things yourself. Written for UK cars, with the figures taken from the fitments we've verified.

Bulbs and lights

How to read a car bulb codeCar bulb codes explained: H means a halogen headlamp type, W a glass wedge, P a metal bayonet cap, Y amber, and a slash such as P21/5W means two filaments.H7 vs H4 vs H1: which headlight bulb does your car use?H7 vs H4: an H7 has one filament for one beam, while an H4 does dipped and main beam in one bulb. H1 is a single-filament main beam bulb. None of them swap.What is an H15 bulb? Main beam and DRL in oneAn H15 bulb is a twin-filament 55/15 W halogen bulb: 55 W for main beam and 15 W for the daytime running light. If one filament fails, replace the bulb.Are LED headlight bulbs legal in the UK?Are LED headlight bulbs legal in the UK? Factory LED headlights are, but LED bulbs fitted in a halogen headlamp are an MOT fail on cars first used from 1986.Should you replace headlight bulbs in pairs?Yes, replace headlight bulbs in pairs: the other bulb has done the same hours and may not be far behind, and a matched pair gives even colour and output.PY21W vs P21W vs WY21W: indicator bulbs explainedPY21W vs P21W: a PY21W is an amber 21 W bayonet bulb with offset pins for clear indicator lenses; a P21W is clear with level pins. WY21W is an amber wedge.Brake light bulb types: P21/5W, P21W and W21/5W explainedMost cars we've verified use a P21/5W twin-filament brake/tail bulb, a single-filament P21W or a W21/5W wedge. Many newer cars have LED rear lights.W5W bulb vs C5W: sidelight and number plate bulbs explainedA W5W is a 5 W glass wedge bulb used in most sidelights and number plate lamps. A C5W is a 5 W festoon, 36 mm long, with a metal cap at each end.When should you use rear fog lights? Rules and bulbsUse rear fog lights only when visibility is seriously reduced, generally under 100 metres, and switch off when it improves. Most take a P21W, W21W or H21W.Front fog light bulb types: H8, H11, HB4 and H16 explainedMost front fog lights we've verified take an H11 (55 W) or H8 (35 W). They share a PGJ19 base family but are keyed differently. HB4, H16 and H3 also appear.LED headlight not working? What to do when a non-replaceable LED failsA non-replaceable LED has no bulb to swap. If an LED headlight or light stops working, the fix is usually a new lamp unit or LED module, fitted by a garage.Halogen, xenon or LED: how to tell if you have xenon headlightsXenon headlights give a bright bluish-white beam that flickers and settles at start-up and use D-coded bulbs. Halogen is warmer and yellower. LED has no bulb.How to change a headlight bulb: a step-by-step guideTo change a headlight bulb, switch off, unplug it, remove the dust cover and clip, fit the new bulb without touching the glass, then test both beams.

Wiper blades

How to measure wiper blades, and why looking up your car is saferMeasure wiper blades tip to tip along the rubber and round to the nearest inch. The old blade may be the wrong size, so check your car's listed size first.Wiper blade connector types: hook, push button, pinch tab and moreThe main wiper blade connector types are hook, push button, pinch tab, side pin, bayonet and top lock. Hook is most common; your wiper arm decides the fit.Flat vs conventional wiper blades: which is better for your car?Flat wiper blades spread pressure through a steel spine with no frame to ice up; conventional blades use a metal frame. Pick the type that fits your arm.Driver and passenger wiper blades on a UK car: how to avoid the LHD trapOn a UK right-hand-drive car the driver's wiper is on the right. Asymmetric blades from LHD sets can sit wrong or lift at speed, so buy a set made for RHD.When to replace wiper blades: 7 signs and what the MOT checksReplace wiper blades when they streak, smear, judder, skip or split, or the rubber goes hard. Yearly is a common rule of thumb; the MOT needs a clear view.Rear wiper blades: sizes, fittings and how to change oneMost rear wiper blades we've verified are 10–13 inches, and 12" is the commonest. Find your car's exact rear blade, its fitting, and how to change it.Why are my wipers streaking or juddering? Causes and fixesWipers streaking usually means a dirty or waxy screen or worn rubber; juddering often means the wrong size or a twisted arm. Here's how to fix each, in order.

Know your car

How to check brake lights on your own, plus every other car lightCheck brake lights on your own by reversing up to a garage door or shop window, propping the pedal down, or filming the back of the car on your phone.Winter visibility checklist: wipers, screenwash and lightsWinter car checklist for UK drivers: winter-strength screenwash, blades in good condition, every bulb working, and all windows clear of ice before you drive.Which generation is my car? Mk numbers, codes and reg platesTo find what generation your car is, match its first-registration date (V5C, reg plate or MOT history) to the model's years, then confirm the Mk or code.How to find your car's bulb and wiper sizes in the handbookYour car handbook bulb chart is usually in the Maintenance, Technical data or Changing bulbs section. Where to look, and what to do if the handbook is lost.