I have a subaru impreza 2.0 gl sport auto and was wondering if anyone could please tell me where the diagnostic plug is on it can't find it any help would be very much appreciated 

What year is it ? If pre 2000 there's two self test diagnostic wires like this

http://uk.subaruownersclub.com/forums/index.php?/topic/2366-Manually-Reading-ECU-Codes

Or if it's later than 2000 it will have an odb2 port just in front of your knee as you sit down in the seat below the steering wheel,

Just to confuse matters older cars also have a hole there but it's for the subaru select monitor which is a dealer only diagnostic tool

It's a 1997 model would that be a dealer diagnostic tool or could we check it ourselfs 

you can check yoursef, If you stick your head under the steering column between the pedals and plastics you should see a forest of wires the self check plugs will be up there, maybe taped up, connect the smaller black ones together and the check engine light will blink out the codes mentioned above in the line if there are any, if not will just flash at a constant rate

Ok thanks we will have a look and see if we can do it 

If's a bit fiddly but easy enough once you've found them