Just carried out the trouble code check on my MY97 impreza by connecting the 2 black wires under the dash.
I managed to read 3 different codes which were:
2 12 43
the list i've got shows 12 as starter switch but has nothing for the rest, any ideas?
I'm guessing the starter switch is down the the clifford alarm.
thanks
Mike
(My97 5dr classic)

 

I thought the 2 wire plug was to check if all was ok whilst you were driving to check all is ok and to resit the ecu.I usually only connect the single wire to read the codes .Take a look at this site the codes are covered in great detail. Are your codes possibly out of sinc ! www.saxonfields.freeserve.co.uk/. You can download a workshop manual from here also.
Good Luck