The fog lights are situated on the front and rear of the vehicle and is used when driving in poor visible conditions such as fog or falling snow or if your visibility drops below 100m. The fog lights on the rear of the vehicle are coloured red, and any fog lights on the front of the vehicle are clear. This is to help other road users work out in which direction a vehicle is travelling.
The fog lights differs to the other lights on the vehicle in that its beams are low and wide to create a narrow bar of light running along the road surface to illuminate any road obstacles. The rear fog lights are used more for the benefit of other drivers behind you so that they are aware of your vehicle and any obstacles between you both.
Because of the intensity of the beam fog lights have restrictions on when they can be used. If used when visibility conditions are not poor, these could temporarily blind other drivers and cause collisions.
Faulty fog lights are an MOT failure and should be corrected quickly.