If you are going to remove your Access Port, and return your ECU to stock, you do not have to take your car to a tuner. You can accomplish this yourself with the Access Port.
To return your ECU to stock, you are going to have to re-write your base map, so you will need to put your car into test mode. Please check Cobb’s instructions that came with your car first.
Basically, you will just need to connect the green “test” connectors under your dash (and the additional connector if you have a 2002 – 2005 WRX). Once you have done this, your Check Engine Light should flash, and you should hear your electric fan under the hood turning on and off.
Once you are in “test” mode, connect your Access Port to your OBDII/CANBUS port and put your key in the on position.
Before you proceed, make sure that you do not have anything pulling current from your electrical system. Make sure that your lights, fan, and stereo are off, that there isn’t anything plugged into you’re lighter socket. If you have a turbo timer, it needs to be turned off, and if you have an aftermarket stereo amplifier, you need to make sure that you pull out the in-line fuse (which is usually next to the battery under the hood).
Once you are ready, use the Access Port to select the “Un-Marry” or “Return to Stock” option from the main menu. You do not want to select a stock map from the “Change Map” menu because this would just put a stock map onto your ECU, while leaving your license on your ECU (so your Access Port will not work with another car).
Once you select this option, let the Access Port run its course, and once it is done, your ECU should be returned to stock.