Mine did this when the CAT was on it's way out! It'll either be the CAT or a lamda. Bear in mind that mine was said to be a lamda when plugged into Reno's computer. As soon as the cat broke in half the light went off! New cat was fitted, no light came back on! For lamdas the best place is clio-spares as they charge £40+Vat for one. Bear in mind there are two lamdas and the are two different part numbers! For a cat, janspeed or k-Tec (circa £300) and a bit of a b**ch to fit!!