I just got 2008 Evo with 20k miles. It has AGP cold air intake, upper intercooler pipe, turboXS downpipe and high flow cat , perrin catback. It has been tuned to 310 wheel hp, fuel trims are within +5%~ -5%, which is good. Only 1or 2 knocks when wide open throttle.
The check engine light got turned on after couple of hundred miles and the owner told me that it's because high flow cat. He had this happen to him 3 times in 15 miles. It drove fine without problems for the rest of my trip (500 miles) with the check engine light. Since then I erased the code once, and it was air flow sensor, and after a few miles upon full throttle it turned up again. As I remember even the first time it got turned on it was during full throttle.
So what would I need to do in order to fix it this. If it helps the air fuel ratio is usually between 14.5 to 16.
Thanks

 

i'd clean your maf to start off and see if that fixes it

 

How can I do that?

 

Can you post the exact code from the CEL? I'm guessing you shouldn't need to clean your MAF, you just need to disable the airflow checks. Please search for more on this topic.

I wouldn't clean the MAF first as this can actually throw it off if it isn't dirty.

 

I am not sure about the code but I did notice that the check engine light turns on seconds after a full throttle. It continues to run fine. I will get the code when I go to see my mechanic, I should get obd 2 scanner.