OBD 2 Reader

hey I just got a OBD 2 reader. The 1st day I plugged it in it showed 5 codes.. most of them EGR and Fuel.. then I cleared it and just yesterday my MIL came back on, I just checked it with the reader now and I only have one code.. which is awkward.. the code is P0400.. which is Exhaust Gas Recirculation Flow. Now a while ago my EGR went bad and was causing my car to stall out so I had the EGR unplugged. What would I have to do in order to get this fixed and his this MIL go off? and how much $ could I be looking at?
(1997 Mazda MX6 LS)

you dont wanna know hom much lol i got a quote to do an egr replacement and it was like 400 ive had my cel on for the an egr related problem going on 4 years now it realy could be anything its prolly not even the egr valve that is bad