I recently had a Perrin Dp and Up installed and i now get a CEL nearing redline in 3rd gear. Never happened before the install. If this has been answered in another location please point that out. I do have plans to either add an AP or possibly a custom tune to the setup. I'm guessing that would solve the problem all together. Just curious on what to do until that time.
You're very likely to throw a code with a downpipe and no tune. Your car is not designed to have additional boost right now, and that is why it's always recommended that you get engine management at the time you get the downpipe. God luck!
Seriously...get a tune and take it easy until you do. The stock map can't keep up with the free flowing exhaust path and you will overboost amongst other things like run too rich/lean (read: not good for the engine).
378awhp/373wtq Custom Dom 3 & CBRD tuned 2006 STI.
"When in doubt...flat out" - Colin McRae