sam i know why your engine light is on its the map or tmap sensor thats the issue. its located behind the alternator between the exhaust manifold pipes it looks like a maf sensor kind of. change that and the light will go out and you will get more power out of the car for sure. had a friend with a focus same engine as yours. change it and you will notice the difference

btw some tips.....

buy a new aux belt because you have to cut off the old one and you need a special tool to put the new one on. you can usually buy it with the belt.

i too hope youve grown up!