I have searched and read a lot of threads about this code. Most are a vaccum leaks and both banks lean. My code checker said only bank one is lean. Also, about a week prior to the light coming on, the 4wd system was malfunctioning. It would not come out of 4x4. Vehicle is a 98 Expedition. I checked the plastic lines by battery and didn't see a problem. Does a 98 have an IWE sensor? What do you experts think could be the problem? Thanks in advance.

 

Yup, sure sounds like a vac leak. Check all the vac lines. If the 4WD is shifted by the vac then you nailed the problem. Being it's a 98 it's probably time to start changing the vac lines any way.