I've installed a downpipe, up pipe, and exhaust on a friends 05wrx. I know that that a cel appears usually after you do this. But in my friends case the cel has not come on. Its been about 2 or so months with the dp and a month or so with the up pipe.

 

I think it depends how much he drives, mine didn't come on for like 500 miles or so

 

depends on the brand of the pipes... if the egt probe is sensitive to probe depth, some pipes throw a cel others don't. if there at least one cat in the downpipe before the o2 sensor, that will not trigger a cel.

 

He has very well put more then enough miles on the down pipe.
As for the brand he has and esx?? i think, down,up pipe and a greddy evo exhaust.