yaris engine check light keeps coming on

[mattuk10]

i have a yaris t sport 2002 and recently the what i think is called the engine check light keeps coming on . it has done it since i owned the car , every couple of months it comes on for a day or two and then goes off. but recently it keeps comin on a lot and comes on for a few days , but sometimes a few hours then goes off , there doesnt seem to b anything wrong with the car it runs starts fine etc , could it be a dodgy sensor ? what makes the sensor come on anyway ?

thanks in advance sorry if this has been posted elsewhere or is in wrong section