My 97 Lhs with check engine light keeps going on &off at times, Checked codes and came up with 12 battery disconnected recently , 21 which looks to be 02 sensor rich or lean other than that It seems to run perfect with 28 mpg. How do I test sensors or should I just replace them both , one before and one after the cat or just one or the other 88,500 mi, recommended replacement according the manual at 100,000mi. Thanks for any help!