2003 Ford Focus 02 sensor on lean

I have posted this problem before it has had this trouble code for a couple of years it does not effect it unless you turn on the ac or the defrosters then it cuts out,but I take that back sometimes it does cut out even though you do not have the ac or defrosters on,it don't stall it just cuts out.But now I am ready to fix it I put new plugs in it a while back and I am going to check fuel pressure.Does anyone know of any other things I could check.