service eng.soon light what can be causing it to come on, fuel cap ?

Get the codes read at the parts store. It could be a bad cap or something else.

Also some cars the light will come on triggered by mileage only, meaning there is not necessarily anything wrong with your car, they just want to get you in the dealer and spend money, but DO get those codes read..and if you write then down and bring then back here we are happy to help. The guys at the parts store, some know what they are talking about and others don't know jack, and don't pay to have them read