If you are doing a lot of short runs there may be a build up of unburnt hydrocarbons and the lambda sensor will register these and switch on the warning lamp.
Try to get a longer run at motorway speeds as this will get the engine up to operating temperature and burn off the unburnt deposits. It also helps to use quality fuel such as Shell V-Power and BP Ultimate rather than cheaper fuels.
If neither of these applies to your car I'd check the air-filter and renew it if it is dirty. A clogged air-filter will cause the engine to run too rich. Incomplete combustion will then throw up the warning lamp.
ChicksFan, March 2013
