This light comes on when there's a fault in the emissions control system. One of the prime causes (but by no means the only one) in a car this age is failure of the airpump, which because of a flawed checkvalve design, fills with water over the years and miles. All this device does is pre-heat the catalytic converter.

There are, of course, other possible causes you'll hear about here. But this is the one that turns on my light from time to time.

David
(98 S70 T5SE Black, misc mods (mostly lighting), red calipers) (92 940GLE)
(Previous: 86 240DL / 88 745T / 94 945T)