On all ODBII cars I have worked on, most older, the downstream O2 is ONLY for verifying that the cats are operating correctly. They have zero impact on fuel trim. Is a 2006 LR3 the same? Or is there some "wide-band" voodoo that is calculated between the upstream and downstream sensors. I am replacing my spark plugs and upstream sensors. Deciding if I should just order downstream as well. Normally I never replace them on a vehicle, being post-cat they last a very long time.