EGR valves hardly ever go bad so get one from the junk yard and get a new gasket from the dealer. The nuts are 12mm, try soaking them with WD-40 then take a punch and wack the nut several times this should help brake it loose if that dont then make sure you have a replacement one first then try bending the EGR valve with a pry bar to one side so you can get the socket on straight as possible, the car should be at operating tempature also. Use a 12mm six point socket.