this is likely the solenoid that controls the egr. follow the tiny hose attached to the egr back to a small electrical solenoid. that part is best bought at a junkyard if possible because it is expensive new.
Not easily, for an idea of the difficulty check out this website (http://www.subarubrat.com/er27install.htm) The guy put the ER27 into a Brat.It can be done though, as I have seen some pictures on youtube (search for ER27 Loyale)
It is possible that a Subaru Check Engine Code #12 is a bad starter switch. This is usually the starter relay switch malfunction code for Subaru.
check driveline, my universal joint went bad and did same, replace and was fine(1990 loyale)
your engine is stuffed
yes
Check this site for information: http://www.troublecodes.net/Subaru/
With the proper scan tool.
there is a emissions related failure on the vehicle.
Not necessarily.
If you take your subaru to Parts plus or an automotive store you can usually have them hook up their computer to the side panel, in the dash board of your car and it will give them a code. The code will tell you exactly why the check engine light is on.
The computer does tests as you drive the car. It has found a problem in a component and is telling you about it. You will have to check for sure but it sounds like code 23 possible Air flow metering problem.
Electronic Controlled Suspension. That's a nice thought, but, I'm pretty sure this is Subaru's version of the 'Check Engine' light. Electronic Stability Control