02 could well be one of the problems mine had 3 patten part ones all failing within 6 months..will casue all sorts of problems,hence i went for O/E part and job done,also check the coolant temp sensor,situated below the alternator,if that's failed it waon't always show a code..hth
02 :- http://www.worldcarparts.co.uk/CarParts/tabid/144/CategoryID/264/Default.aspx
Temp sensor :- http://cgi.ebay.co.uk/Subaru-impreza-turbo-classic-engine-water-temp-sensor-/320669429611?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4aa9632f6b