"aftermarket turbo" Sigh...
You picked up a modded car and are not mechanically inclined
Your modded car started running poorly and you dept driving it
You are asking what it could be when you have a CEL that will tell you?
You are in way, way over your head.
Step 1: GET THAT CEL READ. We need to know what it is.
Step 2: START READING all the stickies in the tuning and turbo subforums
Step 3: order a cable and download software so your laptop can talk to your car
I suggest you find a local subaru tuner shop or post up in your local forum section and meet up with some local members. I hate to think you might toast your car because the previous owner was not kind enough to explain it all to you. Well... buyer beware.