1 have an 06 Mazda 6 with the 3.0 Liter engine. It is starting to slip in 3 gear. It doesn't do it all the time but when I am in the city it is horrible. Also, the check engine light comes on whenever I get gas. A few months ago I changed the gas cap with a locking gas cap because somebody was pouring sugar in the gas tanks in the area. I was told that it is possible that the cap is causing problems with the sensors in the car, but I don't see how it can cause the car to slip in 3rd gear. Does anybody have any suggestions?