engine light *on a timer*

[Karlos2000]

I have an Impreza 1.5 [non turbo],  Engine light came on months ago, and I know it's just an exhaust sensor, 99% of the time, so I waited for the MOT (today), however I was told that the lights come on at certain intervals to remind you to have checked that the cat is functioning properly.  Today they reset the system and it's all fine, passed with no advisories, but was £45 for the reset   

Q : really? they come on at periodic intervals to get the cat checked? if so, how often? 1 year, 2 years, 5 years? Mine is a 2007 really low mileage model.