It's a filter designed to trap excess carbon particles from the un-burnt fuel in the exhaust gases, if you only use your car for short trips or start stop slow driving, this filter can become partialy or even completely blocked, this is when you get the warning light comming on, the best and cheapest fix is to get the car on the motorway and give it a good hard drive using as higher rev's as possible ie keep it in third gear and get up to 70mph for a good few miles and this will cause the filter to get hot and this will dislodge the trapped particles and the light will go out
nigel, June 2010
