I have a check engine light on in my 2010 Ford f150. when I took it into auto zone they gave me a code "P2272 O2 sensor bank 2 sensor 2 signal stuck on lean." The man said it was my o2 sensor that gone bad so, I replace all four. The check engine light is still on and has the same code. A buddy of mine said it could be the EGR valve that needs to be replaced but that part would run me around $120. I don't want to spend that much unless I know it is the problem. Any advice on what it could be?

Thanks!