Lighting: halogen, xenon and LED, factory and replacement
Swapping the bulbs for LEDs is one of the first things anyone thinks of, and one of the things that causes most trouble at the MOT and on the dashboard. Here is what each headlight uses, what can be changed and what cannot, and why the car complains when you fit an LED.
Halogen, xenon and LED
The classic halogen bulb is a glowing filament inside a gas that protects it: cheap, yellowish and using about 55 watts. Xenon has no filament: an electric arc between two electrodes, which needs a ballast to strike it with thousands of volts, gives much more and whiter light. A factory LED gives as much light as xenon or more, uses little and lasts for ages, but the headlight is designed for that specific LED. Illustration 1 shows the headlights and rear lights on.
By regulation, the most powerful headlights, those above 2,000 lumens like almost all xenon ones, must have automatic levelling (which lowers the beam when the car is loaded at the back, so as not to dazzle) and headlight washers.
Replacement LEDs: which ones pass the MOT
Putting an LED bulb in a headlight designed for halogen changes how the light comes out: the headlight is not made for that light source, and the beam dazzles or lights the wrong places. That is why only replacement LEDs approved for your specific model are acceptable: some bulb makers have approval for a list of cars, and it comes with its certificate. If it is not approved for your car, it is not road legal and can fail the MOT. The same goes for xenon kits in halogen headlights.
Why an LED inside the car triggers a bulb failure warning
Many cars monitor their bulbs by measuring how much current they draw. An LED draws so little that the control unit thinks the bulb has blown and warns you on the dashboard, or makes the indicator flash fast. The fix offered by so-called CAN bus LEDs is a resistor that draws the missing current to fool the car. The problem: that resistor turns into heat exactly what the LED was saving, and it gets very hot, hot enough to melt plastic if it sits against it.
The most common faults
- Bulb failure warning with an LED fitted
The car measures the bulb’s current and the LED draws too little. Fixed with an LED with a resistor, minding the heat, or by going back to a normal bulb.
- Indicator flashing very fast
Same reason: the car thinks an indicator bulb has blown.
- Xenon flickering or going out
The ballast or the xenon bulb, which ages and changes colour before it dies.
- Automatic levelling
The rear axle height sensor or the headlight motor. A beam that will not drop or stays down.
- The body control unit is not answering
On many cars it is the one that runs the lights: if it drops out, several things fail at once.
Typical code: U0140
What you can see with Final OBD
On most cars the lights are monitored by the body control unit, not the engine one. With Final OBD you read its faults, each one explained, and if your car gives you a manufacturer code, you can look up what it means in our manufacturer code lookup.
Open the bonnetthe in-depth explanation
How much each one uses and how much light it gives
A 55 watt H7 halogen gives about 1,500 lumens. A 35 watt xenon bulb, about 3,200. A factory LED module, depending on the car, between 1,000 and more than 3,000 per headlight, using far fewer watts. The important figure is not just the total light but where it goes: a good headlight puts the light on the road and cuts it off cleanly at the top so as not to dazzle.
Why the headlight rules
A halogen headlight has a reflector or lens calculated for the filament, which has an exact size and position. An LED has its light points elsewhere and shaped differently, so the beam comes out unfocused: more light where it should not be, glare, and less light far ahead. That is why a replacement LED is approved together with each model’s headlight: a good bulb is not enough.
Levelling and washers: when they are compulsory
European rules require automatic levelling and headlight washers for headlights with light sources above 2,000 lumens. That is why cars with xenon have them, and why fitting xenon to a car that did not come with it is not just a bulb swap. Halogen headlights have manual levelling, the little wheel on the dashboard, to be used when the car is loaded.
Bulb monitoring, inside
The body control unit feeds each bulb through a circuit that measures the current. A 5 watt bulb at 12 volts draws about 0.4 amps; an equivalent LED, a tenth of that or less. Below a threshold, the control unit calls it blown. Some cars also send small test pulses with the light off, and an LED can glow faintly or flicker with those pulses: that is the car checking the bulb.
The resistor maths
For the car to believe there is a 5 watt bulb, the resistor has to burn off almost those 5 watts as heat the whole time the light is on. That is the power of a small soldering iron. On a number plate light or an indicator that is on for short spells, it is fine; on a side light on for hours, inside plastic, it is a risk. If you want LEDs inside, the car maker’s own ones are best, or accept the warning.
Outside, everything approved
Rear lights, indicators, brake lights and number plate lights also have their approval. Swapping a bulb for an LED that is not approved for that lamp can cost you the MOT, and an indicator flashing at double speed is a defect. The safe route is for any change to exterior lighting to carry its approval mark and, where needed, its certificate for your model.
All sections of the mechanics guide
See it in your own car
Almost everything in this guide can be seen with Final OBD and an adapter plugged in: faults explained, live data and the details of each system.
