What model year is your WRX? The '02-'03 0420 CEL fix isn't compatable with the '04s and vise vera. I had really good luck with the Pony Express fix on my '02. I haven't tried the '04 solution yet but I haven't heard anything negative regarding these.

2002-03: http://www.ponyexpressperformance.co...liminator.html

2004: http://www.ponyexpressperformance.co...liminator.html