2006 ford mustang v6 check engine light

[baileysgurl]

I took my Mustang to Firestone in the last week and I just had the thermostat replaced and the radiator cap. They told me the fan could be dying in it as well. I also have the check engine light on. They told me that its the EGR sensor having problems. Every time I go in there keep suggesting for me to get a fuel system tune up and I just got one from them a month ago along with my 90k tune up. The check engine light didn't come on until after I got the tune up. My car has 89,6** miles on it. I'm just wondering what I should do from here and what else it could be. Also every 8 months or so I have to replace the gas cap and I wanted to know if anyone else is having that sort of issue with their car???