I have a 98 F150 with the 5.4 and 150000 km on it. The "check engine" light has come on. I am thinking the O2 sensor is bad. How hard is it to change out or should I go to a mechanic?

 

I changed them 2 weeks ago on my 1998 4.6.. Job took about :45 minutes, if I had to do it again it would probably take :20 minutes.. Really it's not that bad, I found access easiest to get under the truck & then look up on the exhaust pipe & then there right there, & there is a decent amount of room to work on them.. One tip I would suggest is I spent some time trying to break them loose and couldn't so I ran the truck awhile to warm them up, after that they were much easier to get off I also used the special o2 sensor socket worked great

 

Have your codes read to make sure it's the sensor and which one? If it's the same as my 2001 5.4L, it has 4 sensors, 2 before and 2 after the cats. 3 of them are a piece of cake, real easy to get to. The passenger side upstream was a little bit of a PITA because the connector plug is damn hard to get to. It's tucked way up and very liitle room for big hands to get to. They can all be removed with a 7/8" wrench, I didn't need a special socket. As uintahs mentioned above, access is easiest from below and after it's warmed up a little. You don't need a mechanic, can do yourself.